Acidic superabsorbent hydrogels

ABSTRACT

The present invention relates to novel hydrophilic swellable addition polymers comprising improved odor control, a process for their preparation and their use for absorbing aqueous fluids and a process for screening superabsorbents.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 11/145,653,filed Jun. 6, 2005 (now abandoned), which is a divisional of U.S. patentapplication Ser. No. 10/480,980, filed Dec. 15, 2003 (now abandoned),which is the U.S. national phase application of InternationalApplication No. PCT/EP02/06877, filed Jun. 21, 2002, which claims thebenefit of German patent application Nos. 101 30 671.7, filed Jun. 28,2001, 101 42 138.9, filed Aug. 30, 2001, and 101 47 713.9, filed Sept.27, 2001.

The present invention relates to novel hydrophilic swellable additionpolymers comprising improved odor control, their preparation and theiruse for absorbing aqueous fluids and also a process for screeningsuperabsorbents.

More particularly, the present invention relates to acidicsuperabsorbent hydrogels having a pH≦5.7, i.e., hydrogels based onpolyacrylic acid whose degree of neutralization is preferably ≦60 mol %,processes for their preparation and their use in hygiene articles.

Swellable hydrogel forming addition polymers, known as superabsorbentpolymers or SAPs, are known from the prior art. They are networks offlexible hydrophilic addition polymers, which can be both ionic andnonionic in nature. They are capable of absorbing and binding aqueousfluids by forming a hydrogel and therefore are preferentially used formanufacturing tampons, diapers, sanitary napkins, incontinence articles,training pants for children, insoles and other hygiene articles for the,absorption of body fluids. Superabsorbents are also used in other fieldsof technology where fluids, especially water or aqueous solutions, areabsorbed. These fields include for example storage, packaging,transportation (packaging material for water-sensitive articles, forexample flower transportation, shock protection); food sector(transportation of fish, fresh meat; absorption of water, blood in freshfish/meat packs); medicine (wound plasters, water-absorbent material forburn dressings or for other weeping wounds), cosmetics (carrier materialfor pharmaceuticals and medicaments, rheumatic plasters, ultrasound gel,cooling gel, cosmetic thickeners, sunscreen); thickeners for oil/wateror water/oil emulsions; textiles (gloves, sportswear, moistureregulation in textiles, shoe inserts); chemical process industryapplications (catalyst for organic reactions, immobilization of largefunctional molecules (enzymes), adhesive for agglomerations, heatstorage media, filtration aids, hydrophilic component in polymerlaminates, dispersants, liquefiers); building and construction,installation (powder injection molding, clay-based renders,vibration-inhibiting medium, assistants in relation to tunneling inwater-rich ground, cable sheathing); water treatment, waste treatment,water removal (de-icers, reusable sandbags); cleaning; agricultureindustry (irrigation, retention of meltwater and dew precipitates,composting additive, protection of forests against fungal and insectinfestation, delayed release of active ingredients to plants); fireprotection (flying sparks) (covering houses or house walls with SAP gel,since water has a very high heat capacity, ignition can be prevented;spraying of SAP gel in the case of fires such as for example forestfires); coextrusion agent in thermoplastic polymers (hydrophilicizationof multilayer films); production of films and thermoplastic moldingscapable of absorbing water (for example agricultural films capable ofstoring rain and dew water; SAP-containing films for keeping fresh fruitand vegetables which can be packed in moist films; the SAP stores waterreleased by the fruit and vegetables without forming condensationdroplets and partly reemits the water to the fruit and vegetables, sothat neither fouling nor wilting occurs; SAP-polystyrene coextrudatesfor example for food packs such as meat, fish, poultry, fruit andvegetables); carrier substance in active-ingredient formulations (drugs,crop protection). Within hygiene articles, superabsorbents are generallypositioned in an absorbent core which, as well as SAP, comprises othermaterials, including fibers (cellulose fibers), which act as a kind ofliquid buffer to intermediately store the spontaneously applied liquidinsults and are intended to ensure efficient channelization of the bodyfluids in the absorbent core toward the superabsorbent.

The current trend in diaper design is toward ever thinner constructionshaving a reduced cellulose fiber content and an increased hydrogelcontent. The trend toward ever thinner diaper constructions hassubstantially changed the performance profile required of the waterswellable hydrophilic polymers over the years. Whereas at the start ofthe development of highly absorbent hydrogels it was initially solelythe very high swellability on which interest focused, it wassubsequently determined that the ability of the superabsorbent totransmit and distribute fluid is also of decisive importance. It hasbeen determined that conventional superabsorbents greatly swell at thesurface on wetting with liquid, so that transportation of liquid intothe particle interior is substantially compromised or completelyprevented. This trait of superabsorbents is known as gel blocking. Thegreater amount of polymer per unit area in the hygiene article must notcause the swollen polymer to form a barrier layer to subsequent fluid. Aproduct having good transportation properties will ensure optimalutilization of the entire hygiene article. This prevents the phenomenonof gel blocking, which in the extreme case will cause the hygienearticle to leak. Fluid transmission and distribution is thus of decisiveimportance with regard to the initial absorption of body fluids.

Good transportation properties are possessed for example by hydrogelshaving high gel strength in the swollen state. Gels lacking in strengthare deformable under an applied pressure, for example pressure due tothe bodyweight of the wearer of the hygiene article, and clog the poresin the SAP/cellulose fiber absorbent and so prevent continued absorptionof fluid. Enhanced gel strength is generally obtained through a higherdegree of crosslinking, although this reduces retention performance. Anelegant way to enhance gel strength is surface postcrosslinking. In thisprocess, dried superabsorbents having an average crosslink density aresubjected to an additional crosslinking step. The process is known toone skilled in the art and described in EP-A-0 349 240. Surfacepostcrosslinking increases the crosslink density in the sheath of thesuperabsorbent particle, whereby the absorbency under load is raised toa higher level. Whereas the absorption capacity decreases in thesuperabsorbent particle sheath, the core has an improved absorptioncapacity (compared to the sheath) owing to the presence of mobilepolymer chains, so that sheath construction ensures improved fluidtransmission without occurrence of the gel blocking effect. It isperfectly desirable for the total capacity of the superabsorbent to beoccupied not spontaneously but with time delay. Since the hygienearticle is generally repeatedly insulted with urine, the absorptioncapacity of the superabsorbent should sensibly not be exhausted afterthe first disposition.

When hydrogels are used in the hygiene sector, they become exposed tobody fluids such as urine or menses. Body fluids generally containmalodorous components of the amine or fatty acid type, which appearalongside the organic components anyhow present, for example, amines,acids, aldehydes, ketones, phenols, polycyclics, indoles, aromatics,polyaromatics, etc., that are responsible for unpleasant body odors.Odor development takes place in two stages, first in the course ofexudation from the body region and then when the fluid has already beenpresent in the absorption medium for a defined time. Both odor factorshave to be eliminated, since it is undesirable for cost reasons tochange the hygiene article after every absorption process.

The literature on odor control in the hygiene sector reveals thefollowing approaches:

-   -   Odor control coupled with simultaneous absorption by addition of        inert inorganic substances having a large surface area,        generally as a solid onto the surface of powders or granules for        manufacturing absorbent polymers. Zeolites, active carbon,        bentonites, finely divided amorphous silicas such as AEROSIL® or        CAB-O-SIL® are used here.    -   Addition of substances capable of complexing with organic        molecules or with metal ions present in the body fluid to        prevent the development of unpleasant odors. This preferably        takes the form of the use of cyclodextrins (any modification of        unsubstituted cyclodextrins which contains from 6 to 12 glucose        units, for example alpha-cyclodextrin and beta-cyclodextrin,        gamma-cyclodextrin and/or derivatives and/or mixtures thereof.        Mixtures of cyclodextrins are preferred, since they provide        broader complexation of organic molecules over a wider molecular        weight range. Cyclodextrins are used from 0.1% to about 25%,        preferably from 1% to about 20%, more preferably from 2% to        about 15% and especially from 3 to 10%, based on the total        weight of the composition. Cyclodextrins are added in small        particle size (usually less than 12 μm) to offer a large surface        area for odor elimination. Further complexing agents are        aminopolycarboxylic acids and their salts,        ethylenediaminetetraacetate EDTA        ethylenediaminepentamethylenephosphonic acid,        ethylenediaminetetramethylenephosphonic acid, aminophosphates,        polyfunctional aromatics, N,N-disuccinic acid.    -   Masking of unpleasant odors by addition of perfumes or        deodorants. These are added in free form or in encapsulated form        (for example in cyclodextrins). The latter form makes it        possible to release the perfume with a time delay. Nonlimiting        examples of perfumes are allyl caproate, allylcyclohexane        acetate, allylcyclohexane propionate, allyl heptanoate, amyl        acetate, amyl propionate, anetole, anisole, benzaldehyde, benzyl        acetate, benzylacetone, benzyl alcohol, benzyl butyrate, benzyl        formate, benzyl isovalerate, benzyl propionate, butyl benzoate,        butyl caproate, camphor, cis-3-hexenyl acetate, cis-3-hexenyl        butyrate, cis-3-hexenyl caproate, cis-3-hexenyl valerate,        citronellol, citronellyl derivates, Cyclal C, cyclohexylethyl        acetate, 2-decenal, decylaldehyde, dihydromyrcenol,        dimethylbenzylcarbinol and derivatives thereof, dimethyloctanol,        diphenyl oxide, ethyl acetate, ethyl acetoacetate, ethyl amyl        ketone, ethyl benzoate, ethyl butyrate, ethyl hexyl ketone,        ethyl phenylacetate, eucalyptol, fenchyl acetate, fenchyl        alcohol, tricyclodecenyl acetate, tricyclodecenyl propionate,        geraniol, geranyl derivatives, heptyl acetate, heptyl        isobutyrate, heptyl propionate, hexenol, hexenyl acetate,        hexenyl isobutyrate, hexyl acetate, hexyl formate, hexyl        isobutyrate, hexyl isovalerate, hexyl neopentanoate,        hydroxycitronellal, α-ionone, β-ionone, γ-ionone, isoamyl        alcohol, isobornyl acetate, isobornyl propionate, isobutyl        benzoate, isobutyl caproate, isononyl acetate, isononyl alcohol,        isomenthol, isomenthone, isononyl acetate, isopulegol,        isopulegyl acetate, isoquinoline, dodecanal, lavandulyl acetat,        ligustral, δ-limonene, linalool and derivatives, menthone,        menthyl acetate, methylacetophenone, methyl amyl ketone, methyl        anthranilate, methyl benzoate, methyl benzylacetate,        methylchavicol, methyleugenol, methylheptenone, methyl        heptynecarbonate, methyl heptyl ketone, methyl hexyl ketone,        methylnonylacetaldehyde, α-iso“γ”methylionone,        methyloctylacetaldehyde, methyl octyl ketone,        methylphenylcarbinyl acetate, methyl salicylate, myrcene,        myrcenyl acetate, neral, nerol, neryl acetate, nonalactone,        nonyl butyrate, nonyl alcohol, nonyl acetate, nonylaldehyde,        octalactone, octyl acetate, octyl alcohol, octylaldehyde,        d-limonene, p-cresol, p-cresyl methyl ether, p-cymene,        p-isopropyl-p-methylacetophenone, phenethyl anthranilate,        phenoxyethanol, phenylacetaldehyde, phenylethyl acetate,        phenylethyl alcohol, phenylethyldimethylcarbinol, α-pinene,        β-pinene, α-terpinene, γ-terpinene, terpineol, terpinyl acetate,        terpinyl propionate, tetrahydrolinalool, tetrahydromyrcenol,        thymol, prenyl acetate, propyl butyrate, pulegone, safrole,        δ-undecalactone, γ-undecalactone, undecanal, undecyl alcohol,        veratrol, verdox, vertenex, viridine.    -   Addition of urease inhibitors to control the formation or        activity of enzymes responsible for the cleavage of urea into        ammonia and hence for odor development.    -   Addition of antimicrobial substances. Enzymes control bacterial        growth and thereby minimize odor development due to bacterial        degradation processes (e.g., oxidoreductase+mediator). Examples        of antimicrobial substances include quaternary ammonium        compounds, phenols, amides, acids and nitro compounds and also        mixtures thereof.

Examples of quaternary ammonium compounds include2-(3-anilinovinylul)3,4-dimethyloxazolinium iodide, alkylisoquinoliumbromide, benzalkonium chloride, benzethonium chloride, cetylpyridiniumchloride, chlorhexidine gluconate, chlorhexidine hydrochloride,lauryltrimethylammonium compounds, methylbenzethonium chloride,stearyltrimethylammonium chloride, 2,4,5-trichlorophenoxide and alsomixtures thereof.

Examples of phenols include benzyl alcohol, p-chlorophenol,chlorocresol, chloroxylenol, cresol, o-cymen-5-ol (BIOSOL),hexachlorophene, chinokitiol, isopropylmethylphenol, parabens (withmethyl, ethyl, propyl, butyl, isobutyl, isopropyl, and/or sodium methylsubstituents), phenethyl alcohol, phenol, phenoxyethanol,o-phenylphenol, resorcinol, resorcinol monoacetate, sodium parabens,sodium phenolsulfonate, thioxolone, 2,4,4′-trichloro-2′-hydroxydiphenylether, zinc phenolsulfonate, di-tert-butylphenol, hydroquinone, BHT andalso mixtures thereof.

Examples of amides include diazolidinylurea, 2,4-imidazolidinedione(HYDATOIN), 3,4,4′-trichlorocarbanilide,3-trifluoromethyl-4,4′-dichlorocarbanilide, undecylenoic acidmonoethanolamide and also mixtures thereof.

Examples of acids include benzoates, benzoic acid, citric acid,dehydroacetic acid, potassium sorbate, sodium citrates, sodiumdehydroacetate, sodium salicylate, sodium salicylic acid, sorbitanicacid, undecylenoic acid, zinc undecylenate, zinc oxide, zincphenolsulfonate, ascorbic acid, acetylsalicylic acid, salicylaldehyde,salicylic acid derivatives, adipic acid, adipic acid derivatives andalso mixtures thereof.

Examples of nitro compounds include 2-bromo-2-nitro-2,3-propanediol(BRONOPOL), methyldibromoglutaronitrile and propylene glycol (MERGUARD)and also mixtures thereof.

In addition the following compounds are useful as biocides:2,5-dimethoxytetrahydrofuran, 2,5-diethoxytetrahydrofuran,2,5-dimethoxy-2,5-dihydrofuran, 2,5-diethoxy-2,5-dihydrofuran,succinaldehyde, glutaraldehyde, glyoxal, glyoxylic acid,hexahydrotriazine, tetrahydro-3,5-dimethyl-2H-1,3,5-thiadiazine-2-thione(Dazomet), 2,4-dichlorobenzyl alcohol, benzalkonium chloride,chlorhexidine gluconate, triclosan.

-   -   Use of microcapsules which release the active substance on        contact with moisture.    -   Use of transition metal compounds (Cu, Ag, Zn).

As well as the classes of compounds mentioned, useful odor controlcompounds further include the following: peroxides, bicarbonate,triclosan, plant extracts, ethereal oils, boron compounds,poly-alpha-amino acids (polylysine), imides, polyimides, PVP-iodine, useof certain polymeric substances such as chitosan, polyglycosides,oxidizing agents, cyclophanes.

In general, however, the addition of odor inhibitors will have anadverse effect on the absorption profile of superabsorbent hydrogels.The separate installation of the odor-inhibiting or deodorizingcomponent system and of the superabsorbent material in the hygienearticle generally reduces the absorption capacity. Combinationsgenerally exhibit a worse performance profile than the individualcomponents as such. Moreover, the individual components may separateunder mechanical stress of the kind exerted in the course of the wearingof the hygiene article for example. If, however, blends are preparedwhere the additives adhere to the surface of the dried superabsorbentpolymers, the surface properties of the absorbent hydrogels may bechanged without the intrinsic absorption properties being impaired. Theresult may be for example a hydrophilicization or a hydrophobicization,which primarily affects the fluid uptake rate. All these polymers,however, generally have in common that the permeability through swollengel is unsatisfactory.

Odor control on using acidic hydrogels in hygiene articles is good.However, they exhibit a worse absorption profile than is the case withpH neutral products.

The superabsorbent hydrogels used in the hygiene sector at present areaddition polymers possessing a degree of neutralization in the rangefrom 60 to 80 mol %, based on the polymerized acid-functional monomerunits. However, it was found with regard to odor control that a higherpH will generally favor bacterial growth. In the process, the urea inthe urine is increasingly split by urease into carbon dioxide and urea,which leads a further increase in the pH. This in turn reinforcesbacterial growth, and enzyme activity is further increased. Oneconsequence of the raised pH is the occurrence of soft skin, making theskin more susceptible to bacterial colonization. This results directlyin skin irritation which will preclude the wearing of the hygienearticle for a prolonged period.

The manufacturing process of completely acidic hydrogel forming monomersis known and has been repeatedly described in the literature. EP 205 674A1 discloses the preparation of completely acidic addition polymers attemperatures from 0 to 100° C., and preferably from 5 to 40° C., whichare adjusted by subsequent partial neutralization of the hydrogels. Theaddition polymers are notable for improved absorption capacity and alsofor lower extractables. Similarly, U.S. Pat. No. 5,145,906 and EP 530438 B1 disclose the preparation of addition polymer gels from acrylicacid with polymers containing water-soluble hydroxyl groups in an acidicpolymerization, i.e., without neutralization of the monomers, which gelsare subsequently comminuted and partially or completely neutralized bymeans of aqueous bases and subsequently subjected to postcrossslinking.However, the processes all have in common that the polymerization of themonomer solution as shown in EP 467 073 A1 proceeds very slowly, so thatonly a batch process is possible. Increasing the amount of initiator orraising the polymerization temperature has an adverse effect on theabsorption profile of the hydrogels. Moreover, there are appreciableproblems during the manufacturing process with the subdivision of thecompletely acidic polymer gel, and the neutralization which is carriedout subsequently merely takes place under diffusion control, so that thepolymer surface has a base excess. Hydrogels prepared by acidicpolymerization generally exhibit worse absorbencies under load and alsoan appreciable rewet, and this has an adverse effect on the use in thehygiene sector.

On the other hand, there are processes in existence where the monomersolution has already been subjected to a partial neutralization andwhose addition polymer gels are lastly adjusted to the desired degree ofneutralization following the polymerization. For instance, DE 195 29 348reports a process wherein the monomer solution is 5-30 mol %, preferably5-20 mol % and particularly preferably 5-10 mol % neutralized, based onthe acid-functional monomers, whereupon the partially neutralizedmonomer batch is polymerized and subsequently the addition polymer isfurther neutralized until at least 50 mol % of the acid groups presenttherein are neutralized. This process provides addition polymers havinga high retention value and a high sorbency under constant and increasingpressure and also having a low level of extractables. EP 0 583 178 B1,in contrast, proposes a process for preparing superabsorbent powdersconsisting of partially neutralized polyacrylic acids by a sequentialinverse suspension polymerization of two charges having differentdegrees of neutralization (Charge I: degree of neutralization 90-100%,Charge III: degree of neutralization 50-60%), charge II being absorbedbefore polymerization by the polymer of charge I.

None of the cited processes generates hydrogel forming addition polymerswhich confer all the absorption profile advantages of the optimized skinpH neutral superabsorbent on acidic addition polymers, so that adistinct odor control unit is required in each case.

GB 2326348 A and WO 00/35502 impressively report the relationshipbetween bacterial growth and the pH of the surrounding medium. Effectivecontrol of bacterial growth is said to be possible only at pH valuesbetween 3.5 and 4.9, preferably between 4.1 and 4.7. This wouldcorrespond to a degree of neutralization in the range from 20 to 35 mol%. However, these references disregard the quantification of theabsorption profile underlying hydrogels of this degree ofneutralization. GB 2326348 A mentions in general that the lowering ofthe absorption capacity may be compensated by increasing the amount ofhydrogel. WO 00/35502, in contrast, proposes installing the hydrogelforming substance in a layer within the hygiene article that is closerto the skin.

It is an object of the present invention to provide modifiedsuperabsorbents (and a process for their preparation) which when used inhygiene articles comprise improved odor inhibition coupled with theexcellent absorption profile on the part of the superabsorbent hydrogelmaterial. This hydrogel material shall preferably possess rapid swellingand good transportation properties coupled with a high ultimateabsorption capacity and also improved gel strength and higherelectrolyte tolerance without exhibiting the adverse effect of gelblocking. The high absorption performance sought shall not besubstantially impaired by the odor control system, as has heretoforebeen the case in the prior art with odor control in the hygiene sector.

We have found that this object is achieved, surprisingly, preferably bythe sole use of superabsorbent hydrogels whose acid groups have beenneutralized to a lesser degree by the addition of bases before or afterthe polymerization. The range for the degree of neutralization extendsfrom 5 to 60 mol %, preferably from 10 to 40 mol % and particularlypreferably from 20 to 30 mol %, based on the acid-functional monomers.

The present invention accordingly provides for the preparation and useof acidic superabsorbent hydrogels of the above degree of neutralizationwhere preferably no further substances have been added for odorinhibition. Surprisingly, furthermore, we have developed a parameter toquantify the absorption profile—namely the pH absorbency indexpH_(AI)—which permits an informative assessment of newly developedhydrogel material. Furthermore, the pH_(AI) parameter makes it possibleto provide a simple test method for screening and optimizing newmaterials by summarizing and weighting the relevant characteristics ofthe superabsorbents.

The preparation and optimization of novel materials increasinglyutilizes parallel syntheses with or without the assistance of robotsynthesizers. The material obtained has to be performance tested inorder that the next generation of materials may be further optimizedwith regard to one or more parameters. The present invention provides inthe pH absorbency index a test parameter which permits simple yetinformative assessment of superabsorbents. Moreover, the determinationof the individual parameters needed to calculate the index can beminiaturized and parallelized, so that high throughput screening (HTS)of the kind familiar in drug and pesticide discovery research can becarried out. The pH, CFC and AUL-0.7 psi indices can either bedetermined as described in the description part or estimated byconducting equivalent determinations. Such equivalents include forexample the calculation of the degree of neutralization of the monomerused, subsequent neutralization of the gel with known amount ofneutralizing agent etc. for the pH; the AUL-0.7 psi can likewise bereplaced by similar tests at other pressures.

The invention accordingly provides a process for screeningsuperabsorbents which comprises determining or estimating the pH, CRCand AUL 0.7 psi of a plurality of absorbent samples and determining orestimating the pH absorbency index therefrom. Repeated (iterative)application of this screening process by varying one or more opposing ornonopposing parameters makes it possible to optimize the superabsorbentthrough varied production processes from each measurement of the pHabsorbency index. The novel superabsorbents optimized by this processare likewise claimed. A method for determining the swellability ofpolymer gels under pressure is described in PCT/EP/01/12959 and can beappropriately adapted. The disclosure content of this PCT specificationis incorporated in the disclosure content of the present invention.

Commercially available superabsorbent material has a pH of 6. It isknown that the absorption capacity of hydrogels is at a maximum at thispH. Moreover, a high swell rate is observed at this pH. However, the pHat this good absorption performance is distinctly above the pH of theskin, so that the skin may be sensitized and irritated.

The absorption performance at pH 6 can be quantified in terms of theabsorbency under load (0.7 psi) (AUL) of 25 g/g for example and acentrifuge retention capacity CRC of 35 g/g for example. Since the abovenumerical values correlate via the pH, they are simple to combine intoone parameter. The starting point for the calculation is the differencein the pH from pH 7:ΔpH=7−pH of product

The parameter is then calculated therefrom as pH absorbencyindex=pH_(AI):pH_(AI)=ΔpH·(AUL_(0.7 psi)+CRC)

It is a good way of quantifying the performance capability of thesuperabsorbent hydrogel material by stating a single numerical value.

Commercially available polymer material with pH=6, AUL_(0.7 psi)=25 g/gand CRC=35 g/g has a pH absorbency index (calculated as above) ofpH_(AI)=(7−6)·(25+35)=60

Since commercial products for the hygiene sector constitute optimizedsuperabsorbent hydrogel material, a summation factor of 60 may beconsidered to be the lower limit for the performance capability of asuperabsorbent product.

It has been determined that conventional commercial hydrogel materialwithout any odor control has a summation factor in the range from 55 to80, predominantly from 60 to 70.

The present invention accordingly provides acidic hydrogel formingpolymers which are capable of absorbing aqueous fluids and whose acidfunctional monomers have been partially neutralized in a small amount bythe addition of bases before or after polymerization. The partialneutralization range extends from 5 to 60 mol %, preferably from 10 to40 mol % and particularly preferably from 20 to 30 mol %, based on theacid functional monomers. More particularly, the present inventionprovides hydrogel forming polymers capable of absorbing fluids, having apH of ≦5.7, and having a pH absorbency index of at least 80.

Acidic hydrogel material according to the invention, in contrast,comprises complete odor control due to effective control of bacterialgrowth and high absorption performance coupled with a summation factorof above 100, for example in the range from 100 to 130.

Experimental Part

Methods of Making

a) Monomers Used

Hydrogel-forming polymers are in particular polymers of (co)polymerizedhydrophilic monomers, graft (co)polymers of one or more hydrophilicmonomers on a suitable grafting base, crosslinked cellulose or starchethers, crosslinked carboxymethylcellulose, partially crosslinkedpolyalkylene oxide or natural products that swell in aqueous fluids, forexample guar derivatives, alginates and carrageenans.

Suitable grafting bases can be of natural or synthetic origin. Examplesare starch, cellulose or cellulose derivatives and also otherpolysaccharides and oligosaccharides, polyvinyl alcohol, polyalkyleneoxides, especially polyethylene oxides and polypropylene oxides,polyamines, polyamides and also hydrophilic polyesters. Suitablepolyalkylene oxides have for example the formula

where

-   R¹ and R² are independently hydrogen, alkyl, alkenyl or acryl,-   X is hydrogen or methyl and-   n is an integer from 1 to 10 000.-   R¹ and R² are each preferably hydrogen, (C₁-C₄)-alkyl,    (C₂-C₆)-alkenyl or phenyl.

Preferred hydrogel-forming polymers are crosslinked polymers having acidgroups which are predominantly in the form of their salts, generallyalkali metal or ammonium salts. Such polymers swell particularlystrongly on contact with aqueous fluids to form gels.

Preference is given to polymers which are obtained by crosslinkingpolymerization or copolymerization of acid-functional monoethylenicallyunsaturated monomers or salts thereof. It is further possible tocopolymerize these monomers without crosslinker and to crosslink themsubsequently.

Examples of such monomers bearing acid groups are monoethylenicallyunsaturated C₃- to C₂₅-carboxylic acids or anhydrides such as acrylicacid, methacrylic acid, ethacrylic acid, α-chloroacrylic acid, crotonicacid, maleic acid, maleic anhydride, itaconic acid, citraconic acid,mesaconic acid, glutaconic acid, aconitic acid and fumaric acid. It isalso possible to use monoethylenically unsaturated sulfonic orphosphonic acids, for example vinylsulfonic acid, allylsulfonic acid,sulfoethyl acrylate, sulfo methacrylate, sulfopropyl acrylate,sulfopropyl methacrylate, 2-hydroxy-3-acryloyloxypropylsulfonic acid,2-hydroxy-3-methacryloyloxypropylsulfonic acid, vinylphosphonic acid,allylphosphonic acid, styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid. The monomers may be usedalone or mixed.

Preferred monomers used are acrylic acid, methacrylic acid,vinylsulfonic acid, acrylamidopropanesulfonic acid or mixtures thereof,for example mixtures of acrylic acid and methacrylic acid, mixtures ofacrylic acid and acrylamidopropanesulfonic acid or mixtures of acrylicacid and vinylsulfonic acid.

To optimize properties, it can be sensible to use additionalmonoethylenically unsaturated compounds which do not bear an acid groupbut are copolymerizable with the monomers bearing acid groups. Suchcompounds include for example the amides and nitriles ofmonoethylenically unsaturated carboxylic acid, for example acrylamide,methacrylamide and N-vinylformamide, N-vinylacetamide,N-methyl-N-vinylacetamide, acrylonitrile and methacrylonitrile. Examplesof further suitable compounds are vinyl esters of saturated C₁- toC₄-carboxylic acids such as vinyl formate, vinyl acetate or vinylpropionate, alkyl vinyl ethers having at least 2 carbon atoms in thealkyl group, for example ethyl vinyl ether or butyl vinyl ether, estersof monoethylenically unsaturated C₃- to C₆-carboxylic acids, for exampleesters of monohydric C₁- to C₁₈-alcohols and acrylic acid, methacrylicacid or maleic acid, monoesters of maleic acid, for example methylhydrogen maleate, N-vinyllactams such as N-vinylpyrrolidone orN-vinylcaprolactam, acrylic and methacrylic esters of alkoxylatedmonohydric saturated alcohols, for example of alcohols having from 10 to25 carbon atoms which have been reacted with from 2 to 200 mol ofethylene oxide and/or propylene oxide per mole of alcohol, and alsomonoacrylic esters and monomethacrylic esters of polyethylene glycol orpolypropylene glycol, the molar masses (M_(n)) of the polyalkyleneglycols being up to 2 000, for example. Further suitable monomers arestyrene and alkyl-substituted styrenes such as ethylstyrene ortert-butylstyrene.

These monomers without acid groups may also be used in mixture withother monomers, for example mixtures of vinyl acetate and 2-hydroxyethylacrylate in any proportion. These monomers without acid groups are addedto the reaction mixture in amounts within the range from 0 to 50% byweight, preferably less than 20% by weight.

Preference is given to crosslinked polymers of monoethylenicallyunsaturated monomers which bear acid groups and which are optionallyconverted into their alkali metal or ammonium salts before or afterpolymerization and of 0-40% by weight, based on their total weight, ofmonoethylenically unsaturated monomers which do not bear acid groups.

Preference is given to crosslinked polymers of monoethylenicallyunsaturated C₃- to C₁₂-carboxylic acids and/or their alkali metal orammonium salts. Preference is given in particular to crosslinkedpolyacrylic acids where 5-30 mol %, preferably 5-20 mol % andparticularly preferably 5-10 mol % of their acid groups, based on themonomers containing acid groups, are present as alkali metal or ammoniumsalts.

Possible crosslinkers include compounds containing at least twoethylenically unsaturated double bonds. Examples of compounds of thistype are N,N′-methylenebisacrylamide, polyethylene glycol diacrylatesand polyethylene glycol dimethacrylates each derived from polyethyleneglycols having a molecular weight of from 106 to 8 500, preferably from400 to 2 000, trimethylolpropane triacrylate, trimethylolpropanetrimethacrylate, ethylene glycol diacrylate, ethylene glycoldimethacrylate, propylene glycol diacrylate, propylene glycoldimethacrylate, butanediol diacrylate, butanediol dimethacrylate,hexanediol diacrylate, hexanediol dimethacrylate, allyl methacrylate,diacrylates and dimethacrylates of block copolymers of ethylene oxideand propylene oxide, polyhydric alcohols, such as glycerol orpentaerythritol, doubly or more highly esterified with acrylic acid ormethacrylic acid, triallylamine, dialkyldiallylammonium halides such asdimethyldiallylammonium chloride and diethyldiallylammonium chloride,tetraallylethylenediamine, divinylbenzene, diallyl phthalate,polyethylene glycol divinyl ethers of polyethylene glycols having amolecular weight of from 106 to 4 000, trimethylolpropane diallyl ether,butanediol divinyl ether, pentaerythritol triallyl ether, reactionproducts of 1 mol of ethylene glycol diglycidyl ether or polyethyleneglycol diglycidyl ether with 2 mol of pentaerythritol triallyl ether orallyl alcohol, and/or divinylethyleneurea. Preference is given to usingwater-soluble crosslinkers, for example N,N′-methylenebisacrylamide,polyethylene glycol diacrylates and polyethylene glycol dimethacrylatesderived from addition products of from 2 to 400 mol of ethylene oxidewith 1 mol of a diol or polyol, vinyl ethers of addition products offrom 2 to 400 mol of ethylene oxide with 1 mol of a diol or polyol,ethylene glycol diacrylate, ethylene glycol dimethacrylate ortriacrylates and trimethacrylates of addition products of from 6 to 20mol of ethylene oxide with 1 mol of glycerol, pentaerythritol triallylether and/or divinylurea. This invention refers to the same degree ofcrosslinking when the molar ratios between acid-functional monomers andcrosslinkers remain constant with regard to ethylenically unsaturateddouble bonds.

Possible crosslinkers also include compounds containing at least onepolymerizable ethylenically unsaturated group and at least one furtherfunctional group. The functional group of these crosslinkers has to becapable of reacting with the functional groups, essentially the acidgroups, of the monomers. Suitable functional groups include for examplehydroxyl, amino, epoxy and aziridino groups. Useful are for examplehydroxyalkyl esters of the abovementioned monoethylenically unsaturatedcarboxylic acids, e.g., 2-hydroxyethyl acrylate, hydroxypropyl acrylate,hydroxybutyl acrylate, hydroxyethyl methacrylate, hydroxypropylmethacrylate and hydroxybutyl methacrylate, allylpiperidinium bromide,N-vinylimidazoles, for example N-vinylimidazole,1-vinyl-2-methylimidazole and N-vinylimidazolines such asN-vinylimidazoline, 1-vinyl-2-methylimidazoline,1-vinyl-2-ethylimidazoline or 1-vinyl-2-propylimidazoline, which can beused in the form of the free bases, in quaternized form or as salt inthe polymerization. It is also possible to use dialkylaminoethylacrylate and dimethylaminoethyl methacrylate, diethylaminoethyl acrylateand diethylaminoethyl methacrylate. The basic esters are preferably usedin quaternized form or as salt. It is also possible to use glycidyl(meth)acrylate, for example.

Useful crosslinkers further include compounds containing at least twofunctional groups capable of reacting with the functional groups,essentially the acid groups, of the monomers. Suitable functional groupswere already mentioned above, i.e., hydroxyl, amino, epoxy, isocyanato,ester, amido and aziridino groups. Examples of such crosslinkers areethylene glycol, diethylene glycol, triethylene glycol, tetraethyleneglycol, polyethylene glycol, glycerol, polyglycerol, triethanolamine,propylene glycol, polypropylene glycol, block copolymers of ethyleneoxide and propylene oxide, ethanolamine, sorbitan fatty acid esters,ethoxylated sorbitan fatty acid esters, trimethylolpropane,pentaerythritol, 1,3-butanediol, 1,4-butanediol, polyvinyl alcohol,sorbitol, starch, polyglycidyl ethers such as ethylene glycol diglycidylether, polyethylene glycol diglycidyl ether, glycerol diglycidyl ether,glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerolpolyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritolpolyglycidyl ether, propylene glycol diglycidyl ether and polypropyleneglycol diglycidyl ether, polyaziridine compounds such as2,2-bishydroxymethylbutanol tris[3-(1-aziridinyl)propionate],1,6-hexamethylenediethyleneurea,diphenylmethanebis-4,4′-N,N′-diethyleneurea, haloepoxy compounds such asepichlorohydrin and α-methylepifluorohydrin, polyisocyanates such as2,4-toluylene diisocyanate and hexamethylene diisocyanate, alkylenecarbonates such as 1,3-dioxolan-2-one and 4-methyl-1,3-dioxolan-2-one,also bisoxazolines and oxazolidones, polyamidoamines and also theirreaction products with epichlorohydrin, also polyquaternary amines suchas condensation products of dimethylamine with epichlorohydrin, homo-and copolymers of diallyldimethylammonium chloride and also homo- andcopolymers of dimethylaminoethyl (meth)acrylate which are optionallyquaternized with, for example, methyl chloride.

Useful crosslinkers further include multivalent metal ions capable offorming ionic crosslinks. Examples of such crosslinkers are magnesium,calcium, barium and aluminum ions. These crosslinkers are used forexample as hydroxides, carbonates or bicarbonates. Useful crosslinkersfurther include multifunctional bases likewise capable of forming ioniccrosslinks, for example polyamines or their quaternized salts. Examplesof polyamines are ethylenediamine, diethylenetriamine,triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine andpolyethyleneimines and also polyamines having molar masses in each caseof up to 4 000 000.

The crosslinkers are present in the reaction mixture for example from0.001 to 20% and preferably from 0.01 to 14% by weight.

a) Free Radical Polymerization

The polymerization is initiated in the generally customary manner, bymeans of an initiator. But the polymerization may also be initiated byelectron beams acting on the polymerizable aqueous mixture. However, thepolymerization may also be initiated in the absence of initiators of theabovementioned kind, by the action of high energy radiation in thepresence of photoinitiators. Useful polymerization initiators includeall compounds which decompose into free radicals under thepolymerization conditions, for example peroxides, hydroperoxides,hydrogen peroxides, persulfates, azo compounds and redox catalysts. Theuse of water-soluble initiators is preferred. In some cases it isadvantageous to use mixtures of different polymerization initiators, forexample mixtures of hydrogen peroxide and sodium peroxodisulfate orpotassium peroxodisulfate. Mixtures of hydrogen peroxide and sodiumperoxodisulfate may be used in any proportion. Examples of suitableorganic peroxides are acetylacetone peroxide, methyl ethyl ketoneperoxide, tert-butyl hydroperoxide, cumene hydroperoxide, tert-amylperpivalate, tert-butyl perpivalate, tert-butyl perneohexanoate,tert-butyl perisobutyrate, tert-butyl per-2-ethylhexanoate, tert-butylperisononanoate, tert-butyl permaleate, tert-butyl perbenzoate,di(2-ethylhexyl)peroxydicarbonate, dicyclohexyl peroxydicarbonate,di(4-tert-butylcyclohexyl)peroxydicarbonate, dimyristylperoxydicarbonate, diacetyl peroxydicarbonate, allyl peresters, cumylperoxyneodecanoate, tert-butyl per-3,5,5-trimethylhexanoate,acetylcyclohexylsulfonyl peroxide, dilauryl peroxide, dibenzoyl peroxideand tert-amyl perneodecanoate. Particularly suitable polymerizationinitiators are water-soluble azo initiators, e.g.,2,2′-azobis(2-amidinopropane)dihydrochloride,2,2′-azobis(N,N′-dimethylene)isobutyramidine dihydrochloride,2-(carbamoylazo)isobutyronitrile,2,2′-azobis[2-(2′-imidazolin-2-yl)propane] dihydrochloride and4,4′-azobis(4-cyanovaleric acid). The polymerization initiatorsmentioned are used in customary amounts, for example in amounts of from0.01 to 5%, preferably from 0.05 to 2.0%, by weight, based on themonomers to be polymerized.

Useful initiators also include redox catalysts. In redox catalysts, theoxidizing component is at least one of the above-specified per compoundsand the reducing component is for example ascorbic acid, glucose,sorbose, ammonium or alkali metal bisulfite, sulfite, thiosulfate,hyposulfite, pyrosulfite or sulfide, or a metal salt, such as iron(II)ions or sodium hydroxymethylsulfoxylate. The reducing component in theredox catalyst is preferably ascorbic acid or sodium sulfite. Based onthe amount of monomers used in the polymerization, from 3×10⁻⁶ to 1 mol% may be used for the reducing component of the redox catalyst systemand from 0.001 to 5.0 mol % for the oxidizing component of the redoxcatalyst, for example.

When the polymerization is initiated using high energy radiation, theinitiator used is customarily a photoinitiator. Photoinitiators includefor example α-splitters, H-abstracting systems or else azides. Examplesof such initiators are benzophenone derivatives such as Michler'sketone, phenanthrene derivatives, fluorene derivatives, anthraquinonederivatives, thioxanthone derivatives, coumarin derivatives, benzoinethers and derivatives thereof, azo compounds such as the abovementionedfree-radical formers, substituted hexaarylbisimidazoles or acylphosphineoxides. Examples of azides are: 2-(N,N-dimethylamino)ethyl4-azidocinnamate, 2-(N,N-dimethylamino)ethyl 4-azidonaphthyl ketone,2-(N,N-dimethylamino)ethyl 4-azidobenzoate, 5-azido-1-naphthyl2′-(N,N-dimethylamino)ethyl sulfone, N-(4-sulfonylazidophenyl)maleimide,N-acetyl-4-sulfonylazidoaniline, 4-sulfonylazidoaniline, 4-azidoaniline,4-azidophenacyl bromide, p-azidobenzoic acid,2,6-bis(p-azidobenzylidene)cyclohexanone and2,6-bis(p-azidobenzylidene)-4-methylcyclohexanone. Photoinitiators, ifused, are customarily used in amounts of from 0.01 to 5% of the weightof the monomers to be polymerized.

The crosslinked polymers are preferably used in partially neutralizedform. The degree of neutralization is preferably in the range from 5 to60 mol %, more preferably in the range from 10 to 40 mol %, particularlypreferably in the range from 20 to 30 mol %, based on the monomerscontaining acid groups. Useful neutralizing agents include alkali metalbases or ammonia/amines. Preference is given to the use of aqueoussodium hydroxide solution, aqueous potassium hydroxide solution orlithium hydroxide. However, neutralization may also be effected usingsodium carbonate, sodium bicarbonate, potassium carbonate or potassiumbicarbonate or other carbonates or bicarbonates or ammonia. Moreoverprimary, secondary and tertiary amines may be used.

Alternatively, the degree of neutralization can be set before, during orafter polymerization in all apparatuses suitable for this purpose.Neutralization can be effected for example directly in a kneader usedfor polymerization. Neutralization is preferably carried out beforepolymerization.

Industrial processes useful for making these products include allprocesses which are customarily used to make superabsorbers, asdescribed for example in Chapter 3 of “Modern Superabsorbent PolymerTechnology”, F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998.

Polymerization in aqueous solution is preferably conducted as a gelpolymerization. It involves 10-70% strength by weight aqueous solutionsof the monomers and optionally of a suitable grafting base beingpolymerized in the presence of a free-radical initiator by utilizing theTrommsdorff-Norrish effect.

The polymerization reaction may be carried out at from 0 to 150° C.,preferably at from 10 to 100° C., not only at atmospheric pressure butalso at superatmospheric or reduced pressure. As is customary, thepolymerization may also be conducted in a protective gas atmosphere,preferably under nitrogen.

By subsequently heating the polymer gels at from 50 to 130° C.,preferably at from 70 to 100° C., for several hours, the performancecharacteristics of the polymers can be further improved.

The following list relates to particularly preferred conditions forproduction processes of gels and of dried gels prior to surfacepostcrosslinking (base polymers). The missing weight percent from 100%are to be made up with water.

Base Polymer 1:

-   Acrylic acid: 25-40% by weight based on batch size, preferably    28-35% by weight, especially about 31% by weight based on batch    size;-   NaOH 50%: 22-30 mol % based on acrylic acid, preferably 22-29 mol %,    more preferably from 23 to 28 mol %, particularly preferably from 24    to 27 mol %, especially about 25.2 mol % based on acrylic acid, a    corresponding neutralization can also be achieved with other    neutralizing agents;-   polyethylene glycol 400 diacrylate: 0.005-1.0% by weight based on    acrylic acid, preferably 0.1-0.4% by weight, particularly preferably    0.15-0.25% by weight, especially about 0.2% by weight based on    acrylic acid;-   sodium persulfate: 0.2-0.4% by weight based on acrylic acid,    preferably 0.25-0.35% by weight, especially about 0.28% by weight    based on acrylic acid;-   ascorbic acid: 0.005-0.006% by weight based on acrylic acid,    preferably 0.0053-0.0058% by weight especially about 0.0056% by    weight based on acrylic acid;-   sorbitan monococoate: from 0 to 0.1% by weight based on acrylic    acid;-   useful crosslinkers further include other crosslinkers having at    least 2 ethylenically unsaturated double bonds, for example ETMPTA    (ethoxylated trimethylolpropane triacrylate (LAROMER® LR 9015 X from    BASF AG) in place of the polyethylene glycol 400 diacrylate):    0.2-0.5% by weight based on acrylic acid; what is decisive is that    approximately the same degree of crosslinking is achieved.

Useful initiators further include other similarly acting systems orindividual components. In the case of radiative initiation, appropriateradiation initiators have to be used.

Temperature at polymerization start: 15-35° C., especially about 25° C.;

The resulting gel 1 is converted by drying into the base polymer 1.

Drying temperature of gel: 120-200° C., preferably 140-180° C.,especially about 160° C.

Base Polymer 2:

-   Acrylic acid: 25-40% by weight based on batch size, preferably    28-35% by weight, especially about 30% by weight based on batch    size;-   NaHCO₃ 23-38% by weight based on acrylic acid, preferably from 27 to    35% by weight, particular preferably from 30 to 33% by weight,    especially about 31.5% by weight based on acrylic acid, a    corresponding neutralization can also be achieved with other    neutralizing agents;-   sorbitan monococoate: from 0 to 0.15% by weight based on acrylic    acid, preferably 0.02-0.1% by weight, especially about 0.065% by    weight based on acrylic acid;-   allyl methacrylate: 0.005-1.0% by weight based on acrylic acid,    preferably 0.1-0.5% by weight, particularly preferably 0.2-0.4% by    weight, especially about 0.3% by weight based on acrylic acid;-   2,2′-azobisamidinopropane dihydrochloride: 0-0.2% by weight based on    acrylic acid, preferably 0.05-0.1% by weight, especially about 0.08%    by weight based on acrylic acid;-   potassium peroxodisulfate: 0.01-0.3% by weight based on acrylic    acid, preferably 0.1-0.2% by weight, especially about 0.167% by    weight based on acrylic acid;-   ascorbic acid: 0.005-0.03% by weight based on acrylic acid,    preferably 0.01-0.02% by weight especially about 0.015% by weight    based on acrylic acid;-   useful crosslinkers further include other crosslinkers having at    least 2 ethylenically unsaturated double bonds, for example ETMPTA    (ethoxylated trimethylolpropane triacrylate (LAROMER® LR 9015 X from    BASF AG) or polyethylene glycol 400 diacrylate): 0.2-0.5% by weight    based on acrylic acid; what is decisive is that approximately the    same degree of crosslinking is achieved.

Useful initiators further include other similarly acting systems orindividual components. In the case of radiative initiation, appropriateradiation initiators have to be used.

Maximum temperature during polymerization: 70-100° C., especially about90° C.;

The resulting gel 2 is converted by drying into the base polymer 2.

Drying temperature of gel: 120-200° C., preferably about 130-150° C.,especially about 135° C.

Base Polymer 3:

-   Acrylic acid: 25-40% by weight based on batch size, preferably    28-35% by weight, especially about 30% by weight based on batch    size;-   LiOH×H₂O: 15-30% by weight based on acrylic acid, preferably from 17    to 25% by weight, particular preferably from 19 to 23% by weight,    epecially about 20.4% by weight based on acrylic acid, a    corresponding neutralization can also be achieved with other    neutralizing agents;-   sorbitan monococoate: from 0 to 0.15% by weight based on acrylic    acid, preferably 0.02-0.1% by weight, especially about 0.065% by    weight based on acrylic acid;-   allyl methacrylate: 0.005-1.0% by weight based on acrylic acid,    preferably 0.1-0.6% by weight, particularly preferably 0.3-0.5% by    weight, especially about 0.4% by weight based on acrylic acid;-   2,2′-azobisamidinopropane dihydrochloride: 0-0.2% by weight based on    acrylic acid, preferably 0.05-0.1% by weight, especially about 0.08%    by weight based on acrylic acid;-   potassium peroxodisulfate: 0.01-0.3% by weight based on acrylic    acid, preferably 0.1-0.2% by weight, especially about 0.167% by    weight based on acrylic acid;-   ascorbic acid: 0.005-0.03% by weight based on acrylic acid,    preferably 0.01-0.02% by weight especially about 0.015% by weight    based on acrylic acid;-   useful crosslinkers further include other crosslinkers having at    least 2 ethylenically unsaturated double bonds, for example ETMPTA    (ethoxylated trimethylolpropane triacrylate (LAROMER® LR 9015 X from    BASF AG) or polyethylene glycol 400 diacrylate): 0.2-0.5% by weight    based on acrylic acid; what is decisive is that approximately the    same degree of crosslinking is achieved.

Useful initiators further include other similarly acting systems orindividual components. In the case of radiative initiation, appropriateradiation initiators have to be used.

Maximum temperature during polymerization: 70-100° C., especially about90° C.;

The resulting gel 3 is converted by drying into the base polymer 3.

Drying temperature of gel: 120-200° C., preferably about 130-150° C.,especially about 140° C.

Base Polymer 4:

-   Acrylic acid: 15-28% by weight based on batch size, preferably    18-24% by weight, especially about 21% by weight based on batch    size;-   NaHCO₃ 35-55% by weight based on acrylic acid, preferably from 40 to    50% by weight, particular preferably from 43 to 46% by weight,    especially about 44.5% by weight based on acrylic acid, a    corresponding neutralization can also be achieved with other    neutralizing agents;-   2-acrylamido-2-methylpropanesulfonic acid: 30-55% by weight based on    acrylic acid, preferably from 35 to, 50% by weight, particularly    preferably from 41 to 45% by weight, especially about 43% by weight    based on acrylic acid;-   sorbitan monococoate: from 0 to 0.15% by weight based on acrylic    acid, preferably 0.02-0.1% by weight, especially about 0.065% by    weight based on acrylic acid;-   allyl methacrylate: 0.005-1.0% by weight based on acrylic acid,    preferably 0.1-0.6% by weight, particularly preferably 0.3-0.5% by    weight, especially about 0.4% by weight based on acrylic acid;-   2,2′-azobisamidinopropane dihydrochloride: 0-0.2% by weight based on    acrylic acid, preferably 0.05-0.1% by weight, especially about 0.08%    by weight based on acrylic acid;-   potassium peroxodisulfate: 0.01-0.3% by weight based on acrylic    acid, preferably 0.1-0.2% by weight, especially about 0.167% by    weight based on acrylic acid;-   ascorbic acid: 0.005-0.03% by weight based on acrylic acid,    preferably 0.01-0.02% by weight especially about 0.015% by weight    based on acrylic acid;-   useful crosslinkers further include other crosslinkers having at    least 2 ethylenically unsaturated double bonds, for example ETMPTA    (ethoxylated trimethylolpropane triacrylate (LAROMER® LR 9015 X from    BASF AG) or polyethylene glycol 400 diacrylate): 0.2-0.5% by weight    based on acrylic acid; what is decisive is that approximately the    same degree of crosslinking is achieved.

Useful initiators further include other similarly acting systems orindividual components. In the case of radiative initiation, appropriateradiation initiators have to be used.

Maximum temperature during polymerization: 70-100° C., especially about90° C.;

-   The resulting gel 4 is converted by drying into the base polymer 4.

Drying temperature of gel: 120-200° C., preferably about 130-150° C.,especially about 140° C.

Base Polymer 5:

-   Acrylic acid: 20-35% by weight based on batch size, preferably    24-28% by weight, especially about 26% by weight based on batch    size;-   NaOH 50%: 30-60 mol % based on acrylic acid, preferably from 40 to    55 mol %, particular preferably from 44 to 51 mol %, especially    about 45 or 50 mol % based on acrylic acid, a corresponding    neutralization can also be achieved with other neutralizing agents;-   ETMPTA: 0.005-1.0% by weight on acrylic acid, preferably 0.01-0.4%    by weight, particularly preferably 0.03-0.2% by weight, especially    about 0.06% by weight based on acrylic acid-   sodium persulfate: 0.2-0.4% by weight based on acrylic acid,    preferably 0.25-0.35% by weight, especially about 0.28% by weight    based on acrylic acid;-   photoinitiators Darocur 1173: Irgacure 651 (2:1 ratio): 0.005-0.1%    by weight based on acrylic acid, preferably 0.01-0.05% by weight    especially about 0.024% by weight based on acrylic acid;-   useful crosslinkers further include other crosslinkers having at    least 2 ethylenically unsaturated double bonds; what is decisive is    that approximately the same degree of crosslinking is achieved.

Useful initiators further include other similarly acting systems orindividual components, the above-indicated system preferably beinginitiated using a UV lamp. In the case of thermal initiation,appropriate initiator systems have to be used.

The gel obtained may before drying be optionally additionally treatedwith 0-0.1% by weight, preferably 0.01-0.05% by weight especially about0.026% by weight of sodium metabisulphite and with 0-0.1% by weight,preferably 0.01-0.05% by weight, especially about 0.02% by weight ofsorbitan monolaurate.

The resulting gel 5 is converted by drying into the base polymer 5.

Drying temperature of gel: 120-200° C., preferably about 130-150° C.,especially about 145° C.

Base Polymer 6:

-   Acrylic acid: 25-40% by weight based on batch size, preferably    28-35% by weight, especially about 30% by weight based on batch    size;-   NaOH 50%: 30-60 mol % based on acrylic acid, preferably from 40 to    55 mol %, particularly preferably from 48 to 52 mol %, especially    about 50 mol % based on acrylic acid, a corresponding neutralization    can also be achieved with other neutralizing agents;-   polyethylene glycol 400 diacrylate: 0.005-1.1% by weight based on    acrylic acid, preferably 0.1-1.0% by weight, particularly preferably    0.3-0.6% by weight, especially about 0.45% by weight based on    acrylic acid;-   sodium persulfate: 0.2-0.4% by weight based on acrylic acid,    preferably 0.25-0.35% by weight, especially about 0.29% by weight    based on acrylic acid;-   ascorbic acid: 0.005-0.01% by weight based on acrylic acid,    preferably 0.006-0.008% by weight especially about 0.007% by weight    based on acrylic acid;-   sorbitan monococoate: from 0 up to 0.1% by weight based on acrylic    acid; especially about 0.08% by weight based on acrylic acid.

Useful crosslinking agents further include other crosslinkers having atleast 2 ethylenically unsaturated double bonds, such as for exampleETMPTA (ethoxylated trimethylolpropane triacrylate (LAROMER® LR 9015 Xfrom BASF AG) in place of the polyethylene glycol 400 diacrylate):0.2-0.5% by weight based on acrylic acid; what is decisive is thatapproximately the same degree of crosslinking is achieved.

Useful initiators further include other similarly acting systems orindividual components. In the case of radiative initiation, appropriateradiation initiators have to be used.

Temperature at polymerization start: 15-35° C., especially about 25° C.;

The resulting gel 6 is converted by drying into the base polymer 6.

Drying temperature of gel: 120-200° C., preferably 140-180° C.,especially about 160° C.

Not only the gels 1-6 but also the base polymers 1-6 are importantintermediates for the preparation of the inventive hydrogel formingpolymers capable of absorbing aqueous fluids. The gels and base polymersmentioned in the claims are preferred.

Especial preference is given to a gel prepared by polymerization ofpartially neutralized acrylic acid with a crosslinking agent, herein

-   (i) the partial neutralization can be effected using NaOH 50%: 20-30    mol % based on acrylic acid, preferably 22-29 mol %, more preferably    from 23 to 28 mol %, particularly preferably from 24 to 27 mol %,    especially about 25.2 mol % based on acrylic acid, a corresponding    neutralization can also be achieved with other neutralizing agents,    -   the crosslinking can be effected using polyethylene glycol 400        diacrylate: 0.005-1.0% by weight based on acrylic acid,        preferably 0.1-0.4% by weight, particularly preferably        0.15-0.25% by weight, especially about 0.2% by weight based on        acrylic acid;    -   or some other crosslinker which produces the same degree of        crosslinking, or-   (ii) the partial neutralization can be effected using NaHCO₃: 23-28%    by weight based on acrylic acid, preferably from 27 to 35% by    weight, particularly preferably from 30 to 33% by weight, especially    about 31.5% by weight based on acrylic acid, a corresponding    neutralization can also be achieved with other neutralizing agents,    -   the crosslinking can be effected using allyl methacrylate:        0.005-1.0% by weight based on acrylic acid, preferably 0.1-0.5%        by weight, particularly preferably 0.2-0.4% by weight,        especially about 0.3% by weight based on acrylic acid;    -   or some other crosslinker which produces the same degree of        crosslinking,    -   or-   (iii) the partial neutralization can be effected using LiOH×H₂O:    15-30% by weight based on acrylic acid, preferably from 17 to 25% by    weight, particularly preferably from 19 to 23% by weight, especially    about 20.4% by weight based on acrylic acid, a corresponding    neutralization can also be achieved with other neutralizing agents,    -   the crosslinking can be effected using allyl methacrylate:        0.005-1.0% by weight based on acrylic acid, preferably 0.1-0.6%        by weight, particularly preferably 0.3-0.5% by weight,        especially about 0.4% by weight based on acrylic acid;    -   or some other crosslinker which produces the same degree of        crosslinking,    -   or-   (iv) the partial neutralization can be effected using NaHCO₃: 35-55%    by weight based on acrylic acid, preferably from 40 to 50% by    weight, particularly preferably from 43 to 46% by weight, especially    about 44.5% by weight based on acrylic acid, a corresponding    neutralization can also be achieved using other neutralizing agents    when    -   2-acrylamido-2-methylpropanesulfonic acid is present at 30-55%        by weight based on acrylic acid, preferably from 35 to 50% by        weight, particularly preferably from 41 to 45% by weight,        especially about 43% by weight based on acrylic acid        additionally to acrylic acid,    -   the crosslinking can be effected using allyl methacrylate:        0.005-1.0% by weight based on acrylic acid, preferably 0.1-0.6%        by weight, particularly preferably 0.3-0.5% by weight,        especially about 0.4% by weight based on acrylic acid,    -   or some other crosslinker which produces the same degree of        crosslinking    -   or-   (V) the partial neutralization can be effected using NaOH 50%: 30-60    mol % based on acrylic acid, preferably from 40 to 55 mol %,    particularly preferably from 45 to 51 mol %, especially about 45 or    50 mol % based on acrylic acid, a corresponding neutralization can    also be achieved with other neutralizing agents,    -   the crosslinking can be effected using ETMPTA: 0.005-1.0% by        weight based on acrylic acid, preferably 0.1-0.4% by weight,        particularly preferably 0.03-0.2% by weight, especially about        0.06% by weight based on acrylic acid; or some other crosslinker        which produces the same degree of crosslinker,    -   or-   (vi) the partial neutralization can be effected using NaOH 50%:    30-60 mol % based on acrylic acid, preferably from 40 to 55 mol %,    particularly preferably from 48 to 52 mol %, especially about 50 mol    % based on acrylic acid, a corresponding neutralization can also be    achieved with other neutralizing agents,    -   the crosslinking can be effected using polyethylene glycol 400        diacrylate: 0.005-1.1% by weight based on acrylic acid,        preferably 0.1-1.0% by weight, particularly preferably 0.3-0.6%        by weight, especially about 45% by weight, based on acrylic        acid, or some other crosslinker which produces the same degree        of crosslinking.

The abovementioned gels are converted by drying at 120° C.-200° C. intothe base polymers according to the present invention. The water contentis then preferably below 5% by weight and especially below 3% by weight.The base polymers can be converted by the hereinbelow preferred surfacepostcrosslinking conditions into the corresponding superabsorbents, theconditions indicated for each base polymer being preferred for thespecific base polymer.

a) Surface Postcrosslinking

Preference is given to hydrogel forming polymers which have beensurface-postcrosslinked. Surface postcrosslinking may be carried out ina conventional manner using dried, ground and classified polymerparticles.

To effect surface postcrosslinking, compounds capable of reacting withthe functional groups of the polymers by crosslinking are applied to thesurface of the hydrogel particles, preferably in the form of an aqueoussolution. The aqueous solution may contain water-miscible organicsolvents. Suitable solvents are alcohols such as methanol, ethanol,i-propanol ethylene glycol, propylene glycol or acetone.

The subsequent crosslinking reacts polymers which have been prepared bythe polymerization of the abovementioned monoethylenically unsaturatedacids and optionally monoethylenically unsaturated comonomers and whichhave a molecular weight of greater than 5 000, preferably greater than50 000, with compounds which have at least two groups reactive towardacid groups. This reaction can take place at room temperature or else atelevated temperatures up to 220° C.

Suitable postcrosslinkers include for example:

-   -   di- or polyglycidyl compounds such as diglycidyl phosphonates or        ethylene glycol diglycidyl ether, bischlorohydrin ethers of        polyalkylene glycols,    -   alkoxysilyl compounds,    -   polyaziridines, aziridine compounds based on polyethers or        substituted hydrocarbons, for example bis-N-aziridinomethane,    -   polyamines or polyamidoamines and their reaction products with        epichlorohydrin,    -   polyols such as ethylene glycol, 1,2-propanediol,        1,4-butanediol, glycerol, methyltriglycol, polyethylene glycols        having an average molecular weight M_(w) of 200-10 000, di- and        polyglycerol, pentaerythritol, sorbitol, the ethoxylates of        these polyols and their esters with carboxylic acids or carbonic        acid such as ethylene carbonate or propylene carbonate,    -   carbonic acid derivatives such as urea, thiourea, guanidine,        dicyandiamide, 2-oxazolidinone and its derivatives,        bisoxazoline, polyoxazolines, di- and polyisocyanates,    -   di- and poly-N-methylol compounds such as, for example,        methylenebis(N-methylolmethacrylamide) or melamine-formaldehyde        resins,    -   compounds having two or more blocked isocyanate groups such as,        for example, trimethylhexamethylene diisocyanate blocked with        2,2,3,6-tetramethylpiperidin-4-one.

If necessary, acidic catalysts may be added, for examplep-toluenesulfonic acid, phosphoric acid, boric acid or ammoniumdihydrogenphosphate.

Particularly suitable postcrosslinkers are di- or polyglycidyl compoundssuch as ethylene glycol diglycidyl ether, the reaction products ofpolyamidoamines with epichlorohydrin and 2-oxazolidinone.

The crosslinker solution is preferably applied by spraying with asolution of the crosslinker in conventional reaction mixers or mixingand drying equipment such as Patterson-Kelly mixers, DRAIS turbulencemixers, Lödige mixers, screw mixers, plate mixers, fluidized bed mixersand Schugi Mix. The spraying of the crosslinker solution may be followedby a heat treatment step, preferably in a downstream dryer, at from 80to 230° C., preferably 80-190° C., particularly preferably at from 100to 160° C., for from 5 minutes to 6 hours, preferably from 10 minutes to2 hours, particularly preferably from 10 minutes to 1 hour, during whichnot only cracking products but also solvent fractions can be removed.But the drying may also take place in the mixer itself, by heating thejacket or by blowing in a preheated carrier gas.

In a particularly preferred embodiment of the invention, thehydrophilicity of the particle surface of the hydrogel-forming polymeris additionally modified by formation of complexes. The formation ofcomplexes on the outer shell of the hydrogel particles is effected byspraying with solutions of divalent or more highly valent metal saltsolutions, and the metal cations can react with the acid groups of thepolymer to form complexes. Examples of divalent or more highly valentmetal cations are Mg²⁺, Ca²⁺, Al³⁺, Sc³⁺, Ti⁴⁺, Mn²⁺, Fe^(2+/3+), Co²⁺,Ni²⁺, Cu^(+/2+), Zn²⁺, Y³⁺, Zr⁴⁺, Ag⁺, La³⁺, Ce⁴⁺, Hf⁴⁺, and Au^(+/3+),preferred metal cations are Mg²⁺, Ca²⁺, Al³⁺, Ti⁴⁺, Zr⁴⁺and La³⁺, andparticularly preferred metal cations are Al³⁺, Ti⁴⁺ and Zr⁴⁺. The metalcations may be used not only alone but also mixed with each other. Ofthe metal cations mentioned, all metal salts are suitable that possessadequate solubility in the solvent to be used. Of particular suitabilityare metal salts with weakly complexing anions such as for examplechloride, nitrate and sulfate. Useful solvents for the metal saltsinclude water, alcohols, DMF, DMSO and also mixtures thereof. Particularpreference is given to water and water-alcohol mixtures such as forexample water-methanol or water-1,2-propanediol.

The spraying of the metal salt solution onto the particles of thehydrogel-forming polymer may be effected not only before but also afterthe surface postcrosslinking of the particles. In a particularlypreferred process, the spraying of the metal salt solution takes placein the same step as the spraying with the crosslinker solution, the twosolutions being sprayed separately in succession or simultaneously viatwo nozzles or the crosslinker and metal salt solutions may be sprayedconjointly through a single nozzle.

Optionally, the hydrogel-forming polymers may be further modified byadmixture of finely divided inorganic solids, for example silica,alumina, titanium dioxide and iron(II) oxide, to further augment theeffects of the surface after treatment. Particular preference is givento the admixture of hydrophilic silica or of alumina having an averageprimary particle size of from 4 to 50 nm and a specific surface area of50-450 m²/g. The admixture of finely divided inorganic solids preferablytakes place after the surface modification throughcrosslinking/complexing, but may also be carried out before or duringthese surface modifications.

The following list relates to particularly preferred postcrosslinkingconditions of dried gels according to the invention, especially toparticularly preferred postcrosslinking conditions of base polymer 1:

-   ethylene glycol diglycidyl ether: 0.01-0.12% by weight based on    dried gel (base polymer), preferably 0.012-0.02% by weight,    especially about 0.015% by weight based on base polymer;-   water: 1.5-5% by weight based on base polymer, preferably 1.6-2% by    weight, especially about 1.67% by weight based on base polymer;-   sorbitan monococoate: 0-0.1% by weight based on base polymer,    preferably 0.03-0.07% by weight, especially about 0.05% by weight    based on base polymer;-   heat treatment jacket temperature: 120-180° C., preferably 140-160°    C., especially 150° C.; heat treatment residence time has to be    conformed to the temperature, higher temperatures involving shorter    residence times and longer residence times giving rise to more    pronounced postcrosslinking. Typical values are 150-10. minutes,    especially about 120 minutes.-   ethylene glycol diglycidyl ether: 0.01-0.12% by weight based on    dried gel (base polymer), preferably 0.08-0.11% by weight,    especially about 0.10% by weight based on base polymer;-   water: 0.5-5% by weight based on base polymer, preferably 1-2% by    weight, especially about 1.5% by weight based on base polymer;-   1,2-propanediol: 0-3.5% by weight based on base polymer, preferably    0.5-1.5% by weight, especially about 0.8% by weight based on base    polymer;-   aluminum sulfate (as 26.8% solution for example): 0-0.15% by weight    based on base polymer, preferably 0.03-0.10% by weight, especially    about 0.075% by weight based on base polymer;-   heat treatment jacket temperature: 120-180° C., preferably 140-160°    C., especially 150° C.; heat treatment residence time has to be    conformed to the temperature, higher temperatures involving shorter    residence times and longer residence times giving rise to more    pronounced postcrosslinking. Typical values are 150-10 minutes,    especially about 15 minutes.-   ethylene glycol diglycidyl ether: 0.01-0.12% by weight based on    dried gel (base polymer), preferably 0.08-0.11% by weight,    especially about 0.10% by weight based on base polymer;-   water: 0.5-5% by weight based on base polymer, preferably 1-2% by    weight, especially about 1.67% by weight based on base polymer;-   heat treatment jacket temperature: 120-180° C., preferably 140-160°    C., especially 150° C.; heat treatment residence time has to be    conformed to the temperature, higher temperatures involving shorter    residence times and longer residence times giving rise to more    pronounced postcrosslinking. Typical values are 150-10 minutes,    especially about 15 minutes.

The longer the postcrosslinking time, the higher the resultant AUL,although CRC decreases somewhat.

The following list relates to particularly preferred postcrosslinkingconditions of dried gels according to the invention, especially toparticularly preferred postcrosslinking conditions of base polymer 2:

-   oxazolidinone: 0.005-0.1% by weight based on dried gel (base    polymer), preferably 0.01-0.05% by weight, especially about 0.025%    by weight based on base polymer;-   water: 0.5-5% by weight based on base polymer, preferably 1-3% by    weight, especially about 2% by weight based on base polymer;-   1,2-propanediol: 0-4% by weight based on base polymer, preferably    0.5-3% by weight, especially about 2% by weight based on base    polymer;-   aluminum sulfate×8H₂O: 0-0.3% by weight based on base polymer,    preferably 0.03-0.10% by weight, especially about 0.05% by weight    based on base polymer;-   heat treatment jacket temperature: 120-180° C., preferably 140-160°    C., especially 150° C.; heat treatment residence time has to be    conformed to the temperature, higher temperatures involving shorter    residence times and longer residence times giving rise to more    pronounced postcrosslinking. Typical values are 150-10 minutes,    preferably 90-20 minutes, especially about 30, 60 or 70 minutes.

The following list relates to particularly preferred postcrosslinkingconditions of dried gels according to the invention, especially toparticularly preferred postcrosslinking conditions of base polymer 3:

-   ethylene glycol diglycidyl ether: 0.005-0.12% by weight based on    dried gel (base polymer), preferably 0.01-0.05% by weight,    especially about 0.03% by weight based on base polymer;-   water: 0.5-5% by weight based on base polymer, preferably 2-4% by    weight, especially about 3.35% by weight based on base polymer;-   1,2-propanediol: 0-4% by weight based on base polymer, preferably    0.5-3% by weight, especially about 1.6% by weight based on base    polymer;-   aluminum sulfate: 0-0.3% by weight based on base polymer, preferably    0.05-0.10% by weight, especially about 0.075% by weight based on    base polymer;-   heat treatment circulating air drying cabinet temperature: 80-180°    C., preferably 9.0-120° C., especially about 100° C.; heat treatment    residence time has to be conformed to the temperature, higher    temperatures involving shorter residence times and longer residence    times giving rise to more pronounced postcrosslinking. Typical    values are 180-20 minutes, preferably 150-90 minutes, especially    about 120 minutes.

The following list relates to particularly preferred postcrosslinkingconditions of dried gels according to the invention, especially toparticularly preferred postcrosslinking conditions of base polymer 4:

-   ethylene glycol diglycidyl ether: 0.005-0.3% by weight based on    dried gel (base polymer), preferably 0.05-0.1% by weight, especially    about 0.075% by weight based on base polymer or preferably 0.1-0.3%    by weight, especially about 0.2% by weight based on base polymer;-   water: 0.5-5% by weight based on base polymer, preferably 1-3% by    weight, especially about 2% by weight or 2.3% by weight based on    base polymer;-   1,2-propanediol: 0-4% by weight based on base polymer, preferably    1-3% by weight, especially about 2% by weight or 1.2% by weight    based on base polymer;-   aluminum sulfate: 0-0.3% by weight based on base polymer, preferably    0.05-0.10% by weight, especially about 0.075% by weight based on    base polymer;-   heat treatment circulating air drying cabinet temperature: 80-180°    C., preferably 100-160° C., especially about 140° C.; heat treatment    residence time has to be conformed to the temperature, higher    temperatures involving shorter residence times and longer residence    times giving rise to more pronounced postcrosslinking. Typical    values are 180-20 minutes, preferably 150-90 minutes, especially    about 120 minutes.

The following list relates to particularly preferred postcrosslinkingconditions of dried gels according to the invention, especially toparticularly preferred postcrosslinking conditions of base polymer 5:

-   ethylene glycol diglycidyl ether: 0.005-0.2% by weight based on    dried gel (base polymer), preferably 0.03-0.1% by weight, especially    about 0.06% by weight based on base polymer;-   water: 0.5-5% by weight based on base polymer, preferably 1-3% by    weight, especially about 2.5% by weight based on base polymer;-   1,2-propanediol: 0-4% by weight based on base polymer, preferably    0.5-3% by weight, especially about 1.5% by weight based on base    polymer;-   heat treatment circulating air drying cabinet temperature: 80-180°    C., preferably 100-160° C., especially about 145° C.; heat treatment    residence time has to be conformed to the temperature, higher    temperatures involving shorter residence times and longer residence    times giving rise to more pronounced postcrosslinking. Typical    values are 180-20 minutes, preferably 90-30 minutes, especially    about 60 minutes.

The following list relates to particularly preferred postcrosslinkingconditions of dried gels according to the invention, especially toparticularly preferred postcrosslinking conditions of base polymer 6:

-   ethylene glycol diglycidyl ether: 0.01-0.12% by weight based on    dried gel (base polymer), preferably 0.04-0.08% by weight,    especially about 0.06% by weight based on base polymer;-   water: 0.5-5% by weight based on base polymer, preferably 2-4% by    weight, especially about 3.2% by weight based on base polymer;-   1,2-propanediol: 0-4% by weight based on base polymer, preferably 2%    by weight, especially about 1.6% by weight based on base polymer 2;-   heat treatment circulating air drying cabinet temperature: 120-180°    C., preferably 140-160° C., especially 150° C.; heat treatment    residence time has to be conformed to the temperature, higher    temperatures involving shorter residence times and longer residence    times giving rise to more pronounced postcrosslinking. Typical    values are 150-10 minutes, especially about 120 minutes.

The longer the postcrosslinking time, the higher the resultant AUL,although CRC decreases somewhat.

Properties of acidic hydrogel forming polymers according to theinvention

The inventive acidic hydrogel forming polymers capable of absorbingaqueous fluids have a particle size distribution which is generally inthe range from 10 μm to about 1000 μm, preferably in the range fromabout 100 μm to about 850 μm and especially in the range from 150 μm toabout 700 μm. The size windows mentioned preferably include more than80% by weight and especially more than 90% by weight of the particles.

The inventive acidic hydrogel forming polymers capable of absorbingaqueous fluids comprise improved odor control properties as well as highultimate absorption capacity, high gel strength and permeability andalso high retention. Owing to the presence of acidic hydrogel formingpolymers, the products of the invention have antimicrobial properties,thereby providing an odor control system without the need for theaddition of odor inhibiting substances or odor masking materials.

In contrast to the prior art, where an added odor control unit isindispensable for the use of superabsorbent polymers in the hygienesector, the products of the invention permit substantially less costlymanufacture, since as well as there being no need for an odor controlunit there is no need either for binders or other aids for binding anodor control unit to hydrogel forming polymers.

The reduction or preferably the elimination of additives for odorcontrol purposes results in no changes to the high absorptionperformance and no changes to the excellent absorption behavior of thehydrogel forming polymer material used. This in turn provides longerwear times when the products of the invention are used in a hygienearticle. Skin sensitization and irritation is completely avoided andeliminated by a constant pH medium.

The pH of the hydrogel forming polymers according to the invention canbe measured by the methods indicated in the description part and is 5.7or less, especially 5.6, 5.5, 5.4, 5.3, 5.2 or 5.1 and less, preferably5.0 especially 4.9, 4.8, 4.7 4.6 and less, particularly preferably 4.5;the lower limit is particularly preferably 4.4 especially 4.3, 4.2 or4.1, preferably 4.0 especially.3.9, 3.8, 3.7, 3.6, 3.5, 3.4, 3.3, 3.2,3.1 or 3.0, preference being given to combinations of the upper andlower limits, for example pH values in the range from 3 to 5.7,preferably in the range from 4 to 5.5 and particularly preferably in therange from 4.4 to 4.6 or from 5.1 to 5.3.

The SFC value [in 10⁻⁷ cm³s/g] of the hydrogel forming polymersaccording to the invention can be measured by the methods indicated inthe description part and is preferably above 1, especially 2, 4, 6, 8,10, 12, 14, 16, 18, 20 or higher, particularly preferably 22, especially24, 26, 28, 30, 32 or higher.

The CRC value [g/g] of the hydrogel forming polymers according to theinvention can be measured by the methods indicated in the descriptionpart and is preferably above 15, especially 16, 18, 20, 22, 24, orhigher, particularly preferably 25, especially 26, 27, 28, 29, 30, 31,32 or higher.

The AUL-0.7 psi value [g/g] of the hydrogel forming polymers accordingto the invention can be measured by the methods indicated in thedescription part and is preferably above 4, especially 6, 8, 10, 12, orhigher, particularly preferably 13, especially 14, 15, 16, 17, 18, 19,20, 21, 22, 23, 24, 25, 26, 27, 28, or higher.

The pH_(AI) value of the hydrogel forming polymers according to theinvention can be measured and calculated by the methods indicated in thedescription part and is at least 80 or higher, especially 81, 82, 83,84, 85, 86, 87, 88, 90 or higher, preferably above 91, especially 92,93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108,109, 110, 111, 112, 113, 114, 115, 116, 117 118, 119, 120 or higher.

The Nessler value (measured as N₂ from NH₃ in mg/1 compared with N₂ fromNH₃ of Example 9. The N₂ value of Example 9 is set at 100%. The hydrogelforming polymer according to the invention can be measured andcalculated by the methods indicated in the description part and is atmost 65% or less, especially 60%, 55%, 50% of the value of HySorb C7015® or less, preferably less than 45%, especially 40%, 39%, 38%, 37%,36%, 35%, 34%, 33%, 32%, 31%, 30%, 29%, 28%, 27%, 26%, 25%, 24%, 23%,22%, 21%, 20% or less.

Particular preference is given to a combination of the threshold values,for example pH_(AI) with pH, pH_(AI) with SFC, pH_(AI) with CRC, pH_(AI)with AUL, pH_(AI) with Nessler, especially triple combinations such aspH_(AI) with pH and SFC, pH_(AI) with pH and CRC, pH_(AI) with pH andAUL, pH_(AI) with Nessler and SFC, pH_(AI) with Nessler and CRC, pH_(AI)with Nessler and AUL.

Use of Acidic Hydrogel Forming Polymers

The present invention further provides for the use of the abovementionedhydrogel forming polymers in hygiene articles comprising

-   (A) a liquid pervious topsheet-   (B) a liquid impervious backsheet-   (C) a core positioned between (A) and (B) and comprising    -   10-100% by weight of the hydrogel forming polymer according to        the invention    -   0-90% by weight of hydrophilic fiber material        -   preferably 20-100% by weight of the inventive hydrogel            forming polymer, 0-80% by weight of hydrophilic fiber.            material        -   more preferably 30-100% by weight of the inventive hydrogel            forming polymer, 0-70% by weight of hydrophilic fiber            material        -   even more preferably 40-100% by weight of the inventive            hydrogel forming polymer, 0-60% by weight of hydrophilic            fiber material        -   much more preferably 50-100% by weight of the inventive            hydrogel forming polymer, 0-50% by weight of hydrophilic            fiber material        -   particularly preferably 60-100% by weight of the inventive            forming polymer, 0-40% by weight of the hydrophilic fiber            material        -   especially preferably 70-100% by weight of the inventive            hydrogel forming polymer, 0-30% by weight of the hydrophilic            fiber material        -   extremely preferably 80-100% by weight of the inventive            hydrogel forming polymer, 0-20% by weight of the hydrophilic            fiber material        -   most preferably 90-100% by weight of the inventive hydrogel            forming polymer, 0-10% by weight of the hydrophilic fiber            material-   (D) optionally a tissue layer positioned directly above and below    said core (C) and-   (E) optionally an acquisition layer positioned between (A) and (C).

The percentages are to be understood so that in the case of 10-100% byweight 11, 12, 13, 14, 15, 16, 17, 18, 19 up to in each case 100% byweight of hydrogel forming polymer according to the invention and all inbetween % ages (for example 12.2%) are possible and correspondinglyhydrophilic fiber material from 0 to respectively 89, 88, 87, 86, 85,83, 82, 81% by weight and in between percentages (for example 87.8%) arepossible. When further materials are present in the core, then thepercentages of polymer and fiber are reduced accordingly. The sameapplies to the preferred ranges, for example in the case of extremelypreferably 81, 82, 83, 84, 85, 86, 87, 88, 89% by weight can be presentfor the hydrogel forming polymer of the invention and correspondingly19, 18, 17, 16, 15, 14, 13, 12, 11% by weight of the fiber material. Sothe preferred range contains 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 to100% by weight of hydrogel forming polymer according to the invention,the more preferred range 30, 31, 32, 33, 34, 35, 36, 37, 38, 39 to 100%by weight of hydrogel forming polymer according to the invention, theeven more preferred range 40, 41, 42, 43, 44, 45, 46, 47, 48, 49 to 100%by weight of hydrogel forming polymer according to the invention, themuch more preferred range 50, 51, 52, 53, 54, 55, 56, 57, 58, 59 to 100%by weight of hydrogel forming polymer according to the invention, theparticularly preferred range 60, 61, 62, 63, 64, 65, 66, 67, 68, 69 to100% by weight of hydrogel forming polymer according to the invention,the especially preferred range 70, 71, 71, 72, 73, 74, 75, 76, 77, 78,79 to 100% by weight of hydrogel forming polymer according to theinvention and the most preferred range 90, 91, 92, 93, 94, 95, 96, 97,98, 99 or 1.00% by weight of hydrogel forming polymer according to theinvention.

Hygiene articles for the purposes of the present invention include notonly incontinence pads and incontinence briefs for adults but alsodiapers for infants.

The liquid pervious topsheet (A) is the layer which is in direct contactwith the skin of the wearer. Its material comprises customary syntheticor manufactured fibers or films of polyesters, polyolefins, rayon ornatural fibers such as cotton. In the case of non-woven materials thefibers are generally joined together by binders such as polyacrylates.Preferred materials are polyesters, rayon or blends thereof,polyethylene and polypropylene. Examples of liquid pervious layers aredescribed in WO 99/57355 A1, EP 102 388 3 A2.

The liquid impervious layer (B) is generally a sheet of polyethylene orpolypropylene.

The core (C) includes not only the hydrogel forming polymer of theinvention but also hydrophilic fiber material. By hydrophilic is meantthat aqueous fluids spread quickly over the fiber. The fiber material isusually a cellulose, modified cellulose, rayon, polyester such aspolyethylene terephthlate. Particular preference is given to cellulosefibers such as pulp. The fibers generally have a diameter of 1-200 μm,and preferably 10-100 μm, and also have a minimum length of 1 mm.

Diaper construction and shape is common knowledge and described forexample in WO 95/26 209 page 66 line 34 to page 69 line 11, DE 196 04601 A1, EP-A-0 316 518 and EP-A-0 202 127. Diapers and other hygienearticles are-generally also described in WO 00/65084, especially atpages 6-15, WO 00/65348, especially at pages 4-17, WO 00/35502,especially pages 3-9, DE 19737434, WO 98/8439. Hygiene articles forfemcare are described in the following references. The inventivehydrogel forming polymers capable of absorbing aqueous fluids can beused therein. Femcare references: WO 95/24173: Absorption Article forControlling Odour, WO 91/11977: Body Fluid Odour Control, EP 389023:Absorbent Sanitary-Articles, WO 94/25077: Odour Control Material, WO97/01317: Absorbent Hygienic Article, WO 99/18905, EP 834297, U.S. Pat.No. 5,762,644, U.S. Pat. No. 5,895,381, WO 98/57609, WO 2000/065083, WO2000/069485, WO 2000/069484, WO 2000/069481, U.S. Pat. No. 6,123,693, EP1104666, WO 2001/024755, WO 2001/000115, EP 105373, WO 2001/041692, EP1074233. Tampons are described in the following references: WO98/48753., WO 98/41179, WO 97/09022, WO 98/46182, WO 98/46181, WO2001/043679, WO 2001/043680, WO 2000/061052, EP 1108408, WO 2001/033962,DE 200020662, WO 2001/001910, WO 2001/001908, WO 2001/001909, WO2001/001906, WO 2001/001905, WO 2001/24729. Incontinence articles aredescribed in the following references: Disposable Absorbent Article forIncontinent Individuals: EP 311344 description pages 3-9; DisposableAbsorbent Article: EP 850623; Absorbent Article: WO 95/26207; AbsorbentArticle: EP 894502; Dry Laid Fibrous Structure: EP 850 616; WO 98/22063;WO 97/49365; EP 903134; EP 887060; EP 887059; EP 887058; EP 887057; EP887056; EP 931530; WO 99/25284; WO 98/48753. Femcare and incontinencearticles are described in the following references: Catamenial Device:WO 93/22998 description pages 26-33; Absorbent Members for Body Fluids:WO 95/26209 description pages 36-69; Disposable Absorbent Article: WO98/20916 description pages 13-24; Improved Composite AbsorbentStructures: EP 306262 description pages 3-14; Body Waste AbsorbentArticle: WO 99/45973. These references and the references therein arehereby expressly incorporated herein.

The acidic hydrogel forming polymers of the invention are very useful asabsorbents for water and aqueous fluids, so that they may be used withadvantage as a water retainer in market gardening, as a filter aid andparticularly as an absorbent component in hygiene articles such asdiapers, tampons or sanitary napkins.

Incorporation and Fixation of the Highly Swellable Hydrogels Accordingto the Present Invention

In addition to the above-described highly swellable hydrogels, theabsorbent composition of the present invention includes constructionswhich include highly swellable hydrogels or to which they are fixed. Anyconstruction is suitable that is capable of accommodating highlyswellable hydrogels and of being integrated into the absorption layer. Amultiplicity of such compositions is already known and described indetail in the literature. A construction for installing the highlyswellable hydrogels can be for example a fiber matrix consisting of acellulose fiber mixture (air-laid web, wet laid web) or syntheticpolymer fibers (meltblown web, spunbonded web) or else of a fiber blendof cellulose fibers and synthetic fibers. Possible fiber materials aredetailed in the chapter which follows. The air-laid web process isdescribed for example in WO 98/28 478. Furthermore, open-celled foams orthe like may be used to install highly swellable hydrogels.

Alternatively, such a construction can be the result of fusing twoindividual layers to form one or better a multiplicity of chambers whichcontain the highly swellable hydrogels. Such a chamber system isdescribed in detail in EP 0 615 736 A1 page 7 lines 26 et seq.

In this case, at least one of the two layers should be water pervious.The second layer may either be water pervious or water impervious. Thelayer material used may be tissues or other fabric, closed oropen-celled foams, perforated films, elastomers or fabrics composed offiber material. When the absorbent composition consists of aconstruction of layers, the layer material should have a pore structurewhose pore dimensions are small enough to retain the highly swellablehydrogel particles. The above examples of the construction of theabsorbent composition also include laminates composed of at least twolayers between which the highly swellable hydrogels are installed andfixed.

Generally it is possible to fix hydrogel particles within the absorbentcore to improve dry and wet integrity. Dry and wet integrity describesthe ability to install highly swellable hydrogels into the absorbentcomposition in such a way that they withstand external forces not onlyin the wet but also in the dry state and highly swellable polymer doesnot dislocate or spill out. The forces referred to are especiallymechanical stresses as occur in the course of moving about while wearingthe hygiene article or else the weight pressure on the hygiene articlein the case of incontinence especially. As to fixation, one skilled inthe art knows a multiplicity of possibilities. Examples such as fixationby heat treatment, addition of adhesives, thermoplastics, bindermaterials are noted in WO 95/26 209 page 37 line 36 to page 41 line 14.The cited passage is thus part of this invention. Methods for enhancingwet strength are also to be found in WO 2000/36216 A1.

Furthermore, the absorbent composition may comprise a base material, forexample a polymer film on which the highly swellable hydrogel particlesare fixed. The fixing may be effected not only on one side but also onboth sides. The base material can be water pervious or water impervious.

The above constructions of the absorbent composition incorporate thehighly swellable hydrogels at a weight fraction of from 10 to 100% byweight, preferably 20-100% by weight, more preferably 30-100% by weight,even more preferably 40-100% by weight, much more preferably 50-100% byweight, particularly preferably 60-100% by weight, especially preferably70-100% by weight, extremely preferably 80-100% by weight and mostpreferably 90-100% by weight, based on the total weight of theconstruction and of the highly swellable hydrogels.

Fiber Materials of the Absorbent Composition

The structure of the present composition according to the invention maybe based on various fiber materials, which are used as a fiber networkor matrices. The present invention includes not only fibers of naturalorigin (modified or unmodified) but also synthetic fibers.

A detailed overview of examples of fibers which can be used in thepresent invention is given in WO 95/26 209 page 28 line 9 to page 36line 8. The cited passage is thus part of this invention.

Examples of cellulose fibers include cellulose fibers which arecustomarily used in absorption products, such as fluff pulp andcellulose of the cotton type. The materials (soft- or hardwoods),production processes such as chemical pulp, semichemical pulp,chemothermomechanical pulp (CTMP) and bleaching processes are notparticularly restricted. For instance, natural cellulose fibers such ascotton, flax, silk, wool, jute, ethylcellulose and cellulose acetate areused.

Suitable synthetic fibers are produced from polyvinyl chloride,polyvinyl fluoride, polytetrafluoroethylene, polyvinylidene chloride,polyacrylic compounds such as ORLON®, polyvinyl acetate, polyethyl vinylacetate, soluble or insoluble polyvinyl alcohol. Examples of syntheticfibers include thermoplastic polyolefin fibers, such as polyethylenefibers (PULPEX®), polypropylene fibers and polyethylene-polypropylenebicomponent fibers, polyester fibers, such as polyethylene terephthalatefibers (DACRON® or KODEL®), copolyesters, polyvinyl acetate, polyethylvinyl acetate, polyvinyl chloride, polyvinylidene chloride,polyacrylics, polyamides, copolyamides, polystyrene and copolymers ofthe aforementioned polymers and also bicomponent fibers composed ofpolyethylene terephthalate-polyethylene-isophthalate copolymer,polyethyl vinyl acetate/polypropylene, polyethylene/polyester,polypropylene/polyester, copolyester/polyester, polyamide fibers(nylon), polyurethane fibers, polystyrene fibers and polyacrylonitrilefibers. Preference is given to polyolefin fibers, polyester fibers andtheir bicomponent fibers. Preference is further given to thermallyadhesive bicomponent fibers composed of polyolefin of the core-sheathtype and side-by-side type on account of their excellent dimensionalstability following fluid absorption.

The synthetic fibers mentioned are preferably used in combination withthermoplastic fibers. In the course of the heat treatment, the lattermigrate to some extent into the matrix of the fiber material present andso constitute bond sites and renewed stiffening elements on cooling.Additionally the addition of thermoplastic fibers means that there is anincrease in the present pore dimensions after the heat treatment hastaken place. This makes it possible, by continuous addition ofthermoplastic fibers during the formation of the absorbent core, tocontinuously increase the fraction of thermoplastic fibers in thedirection of the topsheet, which results in a similarly continuousincrease in the pore sizes. Thermoplastic fibers can be formed from amultiplicity of thermoplastic polymers which have a melting point ofless than 190° C., preferably in the range from 75° C. to 175° C. Thesetemperatures are too low for damage to the cellulose fibers to belikely.

Lengths and diameters of the above-described synthetic fibers are notparticularly restricted, and generally any fiber from 1 to 200 mm inlength and from 0.1 to 100 denier (gram per 9 000 meters) in diametermay preferably be used. Preferred thermoplastic fibers are from 3 to 50mm in length, particularly preferred thermoplastic fibers are from 6 to12 mm in length. The preferred diameter for the thermoplastic fiber isin the range from 1.4 to 10 decitex, and the range from 1.7 to 3.3decitex (gram per 10 000 meters) is particularly preferred. The form ofthe fiber may vary; examples include woven types, narrow cylindricaltypes, cut/chopped yarn types, staple fiber types and continuousfilament fiber types.

The fibers in the absorbent composition of the present invention can behydrophilic and/or hydrophobic. According to the definition of Robert F.Gould in the 1964 American Chemical Society publication “Contact angle,wettability and adhesion”, a fiber is referred to as hydrophilic whenthe contact angle between the liquid and the fiber (or the fibersurface) is less than 90° or when the liquid tends to spreadspontaneously on the same surface. The two processes are generallycoexistent. Conversely, a fiber is termed hydrophobic when a contactangle of greater than 90° is formed and no spreading is observed.

Preference is given to using hydrophilic fiber material. Particularpreference is given to using fiber material which is weakly hydrophilicon the body side and most hydrophilic in the region surrounding thehighly swellable hydrogels. In the manufacturing process, layers havingdifferent hydrophilicities are used to create a gradient which channelsimpinging fluid to the hydrogel, where it is ultimately absorbed.

Suitable hydrophilic fibers for use in the absorbent composition of thepresent invention include for example cellulose fibers, modifiedcellulose fibers, rayon, polyester fibers, for example polyethyleneterephthalate (DACRON®), and hydrophilic nylon (HYDROFIL®). Suitablehydrophilic fibers may also be obtained by hydrophilicizing hydrophobicfibers, for example the treatment of thermoplastic fibers obtained frompolyolefins (e.g. polyethylene or polypropylene, polyamides,polystyrenes, polyurethanes, etc.) with surfactants or silica. However,for cost reasons and ease of availability, cellulosic fibers arepreferred.

The highly swellable hydrogel particles are embedded into the fibermaterial described. This can be done in various ways, for example byusing the hydrogel material and the fibers together to create anabsorbent layer in the form of a matrix, or by incorporating highlyswellable hydrogels into fiber mixture layers, where they are ultimatelyfixed, whether by means of adhesive or lamination of the layers.

The fluid-acquiring and -distributing fiber matrix may comprisesynthetic fiber or cellulosic fiber or a mixture of synthetic fiber andcellulosic fiber, in which case the mixing ratio may vary from (100 to0) synthetic fiber: (0 to 100) cellulosic fiber. The cellulosic fibersused may additionally have been chemically stiffened to increase thedimensional stability of the hygiene article.

The chemical stiffening of cellulosic fibers may be provided indifferent ways. A first way of providing fiber stiffening is by addingsuitable coatings to the fiber material. Such additives include forexample polyamide-epichlorohydrin coatings (Kymene® 557 H, Hercoles,Inc. Wilmington, Del.), polyacrylamide coatings (described in U.S. Pat.No. 3,556,932 or as the Parez® 631 NC commercial product from AmericanCyanamid Co., Stamford, Conn.), melamine-formaldehyde coatings andpolyethyleneimine coatings.

Cellulosic fibers may also be chemically stiffened by chemical reaction.For instance, suitable crosslinker substances may be added to effectcrosslinking taking place within the fiber. Suitable crosslinkersubstances are typical substances used for crosslinking monomersincluding but not limited to C₂-C₈-dialdehydes, C₂-C₈-monoaldehydeshaving acid functionality and in particular C₂-C₉-polycarboxylic acids.Specific substances from this series are for example glutaraldehyde,glyoxal, glyoxylic acid, formaldehyde and citric acid. These substancesreact with at least 2 hydroxyl groups within any one cellulose chain orbetween two adjacent cellulose chains within any one cellulose fiber.The crosslinking causes a stiffening of the fibers, to which greaterdimensional stability is imparted as a result of this treatment. Inaddition to their hydrophilic character, these fibers exhibit uniformcombinations of stiffening and elasticity. This physical property makesit possible to retain the capillary structure even under simultaneouscontact with fluid and compressive forces and to prevent prematurecollapse.

Chemically crosslinked cellulose fibers are known and described in WO91/11162, U.S. Pat. Nos. 3,224,926, 3,440,135, 3,932,209, 4,035,147,4,822,453, 4,888,093, 4,898,642 and U.S. Pat. No. 5,137,537. Thechemical crosslinking imparts stiffening to the fiber material, which isultimately reflected in improved dimensional stability for the hygienearticle as a whole. The individual layers are joined together by methodsknown to one skilled in the art, for example intermelting by heattreatment, addition of hot-melt adhesives, latex binders, etc.

Methods of Making the Absorbent Composition

The absorbent composition is composed of constructions which containacidic highly swellable hydrogels and the acidic highly swellablehydrogels which are present in said constructions or fixed thereto.

Examples of processes to obtain an absorbent composition comprising forexample a base material to which highly swellable hydrogels are fixed onone or both sides are known and included by the invention but notlimited thereto.

Examples of processes to obtain an absorbent composition comprising forexample highly swellable hydrogels (c) embedded in a fiber materialblend of synthetic fibers (a) and cellulose fibers (b), the blend ratiovarying from (100 to 0) synthetic fiber: (0 to 100) cellulose fiber,include (1) a process where (a), (b) and (c) are mixed together at oneand the same time, (2) a process where a mixture of (a) and (b) is mixedinto (c), (3) a process where a mixture of (b) and (c) is mixed with(a), (4) a process where a mixture of (a) and (c) is mixed into (b), (5)a process where (b) and (c) are mixed and (a) is continuously meteredin, (6) a process where (a) and (c) are mixed and (b) is continuouslymetered in, and (7) a process where (b) and (c) are mixed separatelyinto (a). Of these examples, processes (1) and (5) are preferred. Theapparatus used in this process is not particularly restricted and anycustomary apparatus known to one skilled in the art can be used.

The absorbent composition obtained in this way can optionally besubjected to a heat treatment, so that an absorption layer havingexcellent dimensional stability in the moist state is obtained.

The heat treatment process is not particularly restricted. Examplesinclude heat treatment by feeding hot air or infrared irradiation. Thetemperature of the heat treatment is in the range from 60° C. to 230°C., preferably from 100° C. to 200° C., particularly preferably from100° C. to 180° C.

The duration of the heat treatment depends on the type of syntheticfiber, its amount and the hygiene article production rate. Generally theduration of the heat treatment is in the range from 0.5 second to 3minutes, preferably from 1 second to 1 minute.

The absorbent composition is generally provided for example with aliquid-pervious topsheet and a liquid-impervious backsheet. Furthermore,leg cuffs and adhesive tabs are attached to finalize the hygienearticle. The materials and types of pervious topsheet and imperviousbacksheet and of the leg cuffs and adhesive tabs are known to oneskilled in the art and are not particularly restricted. Examples thereofmay be found in WO 95/26 209.

Experimental Part

Test Methods

a) Centrifuge Retention Capacity (CRC)

This method measures the free swellability of the hydrogel in a teabag.0.2000±0.0050 g of dried hydrogel (particle size fraction 106-850 μm)are weighed into a teabag 60×85 mm in size which is subsequently sealed.The teabag is placed for 30 minutes in an excess of 0.9% by weightsodium chloride solution (at least 0.83 l of sodium chloride solution/1g of polymer powder). The teabag is then centrifuged for 3 minutes at250 g. The amount of liquid is determined by weighing back thecentrifuged teabag.

b) Absorbency Under Load (AUL) (0.7 psi)

The measuring cell for determining AUL0.7 psi is a Plexiglass cylinder60 mm in internal diameter and 50 mm in height. Adhesively attached toits underside is a stainless steel sieve bottom having a mesh size of 36μm. The measuring cell further includes a plastic plate having adiameter of 59 mm and a weight which can be placed in the measuring celltogether with the plastic plate. The plastic plate and the weighttogether weigh 1 345 g. AUL 0.7 psi is determined by determining theweight of the empty Plexiglass cylinder and of the plastic plate andrecording it as W₀. 0.900±0.005 g of hydrogel forming polymer (particlesize distribution 150-800 μm) is then weighed into the Plexiglasscylinder and distributed very uniformly over the stainless steel sievebottom. The plastic plate is then carefully placed in the Plexiglasscylinder, the entire unit is weighed and the weight is recorded asW_(a). The weight is then placed on the plastic plate in the Plexiglasscylinder. A ceramic filter plate 120 mm in diameter and 0 in porosity isthen placed in the middle of a Petri dish 200 mm in diameter and 30 mmin height and sufficient 0.9% by weight sodium chloride solution isintroduced for the surface of the liquid to be level with the filterplate surface without the surface of the filter plate being wetted. Around filter paper 90 mm in diameter and <20 μm in pore size. (S&S 589Schwarzband from Schleicher & Schüll) is subsequently placed on theceramic plate. The Plexiglass cylinder containing hydrogel formingpolymer is then placed with plastic plate and weight on top of thefilter paper and left there for 60 minutes. At the end of this period,the complete unit is removed from the Petri dish and subsequently theweight is removed from the Plexiglass cylinder. The Plexiglass cylindercontaining swollen hydrogel is weighed together with the plastic plateand the weight recorded as W_(b).

AUL was calculated by the following equation:AUL0.7 psi[g/g]=[W _(b) −W _(a) ]/[W _(a) −W ₀]c) Saline Flow Conductivity (SFC)

The test method for determining SFC is described in U.S. Pat. No.5,599,335.

d) pH Measurement of Hydrogel Forming Polymers

100 ml of 0.9% by weight NaCl solution is magnetically stirred atmoderate speed in a 150 ml beaker without air being drawn into thesolution. This solution is admixed with 0.5±0.001 g of hydrogel formingpolymer and stirred for 10 minutes. After 10 minutes, the pH of thesolution is measured with a pH glass electrode, the value not being readoff until it is stable, but at the earliest after 1 minute.

e) Ammonia Determination for Odor Control

The ammonia nitrogen content is determined colorimetrically by theNessler method. Urea eliminates ammonia under the action of urease; ayellow color develops to a degree proportional to the ammoniaconcentration.

5 g of the various superabsorbent samples were saturated with 600 ml of0.9% NaCl and 1.8% urea solution for 20. min. The solutions werefiltered and 25 ml of the solution were admixed with 10 μl of ureasesolution. After 2 minutes nitrogen from ammonia was determined by theNessler method.

EXAMPLES Example 1a

A Werner & Pfleiderer laboratory kneader having a working capacity of 2l is evacuated to 980 mbar absolute by means of a vacuum pump and apreviously separately prepared monomer solution which has been cooled toabout 25° C. and inertized by passing nitrogen into it is sucked intothe kneader. The monomer solution has the following composition: 825.5 gof completely ion-free water, 431 g of acrylic acid, 120.68 g of NaOH50%, 0.86 g of polyethylene glycol 400 diacrylate (SARTOMER® 344 fromCray Valley). To improve the inertization, the kneader is evacuated andsubsequently refilled with nitrogen. This operation is repeated threetimes. A solution of 1.2 g of sodium persulfate (dissolved in 6.8 g ofcompletely ion-free water) is then sucked in, followed after a further30 seconds by a further solution consisting of 0.024 g of ascorbic aciddissolved in 4.8 g of completely ion-free water. After a nitrogen purgea preheated jacket heating circuit on bypass at 75° C. is switched overto the kneader jacket and the stirrer speed increased to 96 rpm.Following the onset of polymerization and the attainment of T_(max), thejacket heating circuit is switched back to bypass, and the batch issupplementarily polymerized for 15 minutes without heating/cooling,subsequently cooled and discharged. The resultant gel particles aredried at 160° C. on wire mesh bottomed trays in a through air dryingcabinet and then ground and sieved.

Example 1b

1 200 g of the thus obtained product of particle size distribution105-850 μm were sprayed with a homogeneous solution consisting of 20 gof water, 0.18 g of ethylene glycol diglycidyl ether and 0.6 g ofsorbitan monococoate in a powder mixing assembly (Loedige mixer) andtransferred into a preheated and 2nd Loedige mixer. The heat treatmentwas carried out under constant conditions at a jacket temperature of150° C. and a speed of 60 rpm for a period of 120 minutes. The mixer wasemptied, and the product was cooled down to room temperature and sievedoff at 105/850 μm to remove any agglomerates or fines which may haveformed. The performance data are shown in Table 1.

Example 1c

Example 1b was repeated except that the heat treatment was carried outfor 70 minutes only and the postcrosslinking solution for 1 200 g ofpowder from Example 1a had the following composition: 17.58 g of water,9.96 g of 1,2-propanediol, 1,2 g of ethylene glycol diglycidyl ether and3.36 g of aqueous 26.8% aluminum sulfate solution. The performance dataare shown in Table 1.

Example 1d

Example 1b was repeated except that the heat treatment was carried outfor 15 minutes only and the postcrosslinking solution for 1 200 g ofpowder from Example la had the following composition: 20.00 g of waterand 0.90 g of ethylene glycol diglycidyl ether. The performance data areshown in Table 1.

Example 2

A 10 1 capacity polyethylene vessel thoroughly insulated with foamedplastic material is charged with 3 928 g of completely ion-free water,630 g of sodium bicarbonate are suspended in the water, and 2 000 g ofacrylic acid are added with stirring in such a way that the reactionsolution does not foam over as a result of the onset of CO₂ evolution.This is followed by the addition, in succession, of an emulsion of 1.3 gof sorbitan monococoate in 100 g of completely ion-free water and of6.00 g of allyl methacrylate, and the solution is further inertized bypassing nitrogen into it. This is followed by the addition of theinitiator system, consisting of 1.66 g of 2,2′-azobisamidinopropanedihydrochloride (dissolved in 20 g of completely ion-free water), 3.33 gof potassium peroxodisulfate (dissolved in 150 g of completely ion-freewater) and also 0.3 g of ascorbic acid (dissolved in 25 g of completelyion-free water) in succession with stirring. The reaction solution isthen left to stand without stirring. The polymerization which ensues,and in the course of which the temperature rises to about 90° C.,produces a solid gel. This solid gel is mechanically comminuted using ameat grinder, dried at 135° C. in a through air cabinet on VA stainlesssteel wire mesh and then ground and sieved.

Example 2a

Product of Example 2 is postcrosslinked similarly to Example 1b using apostcrosslinking solution which had the following composition for 1 000g of polymer: 19.26 g of water, 19.50 g of 1,2-propanediol, 0.25 g of2-oxazolidinone, 0.49 g of aluminum sulfate octadecahydrate. Theperformance data after a residence time of 30 minutes are shown in Table1.

Example 2b

Product of Example 2 is postcrosslinked similarly to Example 2a. Theperformance data after a residence time of 60 minutes are shown in Table1.

Example 2c

The product of Example 2 is postcrosslinked similarly to Example 2a. Theperformance data after a residence time of 70 minutes are shown in Table1.

Example 3

A 10 l capacity polyethylene vessel thoroughly insulated with foamedplastic material is charged with 4 046 g of completely ion-free water,408 g of lithium hydroxide 1-hydrate are dissolved therein, and 2 000 gof acrylic acid are slowly ,added with stirring. This is followed by theaddition, in succession, of an emulsion of 1.3 g of sorbitan monococoatein 100 g of completely ion-free water and of 8.1 g of allylmethacrylate, and the solution is further inertized by passing nitrogeninto it. This is followed by the addition of the initiator system,consisting of 1.66 g of 2,2′-azobisamidinopropane dihydrochloride(dissolved in 20 g of completely ion-free water), 3.33 g of potassiumperoxodisulfate (dissolved in 150 g of completely ion-free water) andalso 0.3 g of ascorbic acid (dissolved in 25 g of completely ion-freewater) in succession with stirring. The reaction solution is thenallowed to stand without stirring. The polymerization which ensues, andin the course of which the temperature rises to about 90° C., produces asolid gel. This solid gel is mechanically comminuted using a meatgrinder. 150 g of the gel thus comminuted are placed in a metal cylinderhaving an internal diameter of 10 cm and a VA stainless steel wire meshbottom, dried therein under a 140° C. 3.11 m/sec hot air stream attemperatures of . . . in the course of 25 minutes, subsequently groundand sieved.

Example 3a

Product of Example 3 is admixed on a 20 g scale in a Waring blender(modified attachment for kitchen blender) with a surfacepostcrosslinking solution (sprayed from a 2 ml hypodermic) consisting of3.35% of water/1.65% of 1,2-propanediol/0.03% of ethylene glycoldiglycidyl ether and 0.075% of aluminum sulfate (each based on polymer)and heat treated at 100° C. in a through air cabinet for 2 hours. Theperformance data are shown in Table 1.

Example 4

A 10 l capacity polyethylene vessel thoroughly insulated with foamedplastic material is charged with 3 944 g of completely ion-free water,625 g of sodium bicarbonate are suspended in the water, and 1 400 g ofacrylic acid are added with stirring in such a way that the reactionsolution does not foam over as a result of the onset of CO₂ evolution.This is followed by the addition, in succession, of 600 g of2-acrylamido-2-methylpropanesulfonic acid and also of an emulsion of 1.3g of sorbitan monococoate in 100 g of completely ion-free water and of6.5 g of allyl methacrylate, and the solution is further inertized bypassing nitrogen into it. This is followed by the addition of theinitiator system, consisting of 1.66 g of 2,2′-azobisamidinopropanedihydrochloride (dissolved in 20 g of completely ion-free water), 3.33 gof potassium peroxodisulfate (dissolved in 150 g of completely ion-freewater) and also 0.3 g of ascorbic acid (dissolved in 25 g of completelyion-free water) in succession with stirring. The reaction solution isthen left to stand without stirring. The polymerization which ensues,and in the course of which the temperature rises to about 90° C.,produces a solid gel. This solid gel is mechanically comminuted using ameat grinder. 150 g of the gel thus comminuted are placed in a metalcylinder having an internal diameter of 10 cm and a VA stainless steelwire mesh bottom, dried therein under a 140° C. 3.1 m/sec hot air streamat temperatures of . . . in the course of 25 minutes, subsequentlyground and sieved.

Example 4a

Product of Example 4 is similarly to Example 3a on a 20 g scale in aWaring blender with a surface postcrosslinking solution consisting of1.95% of water/1.95% of 1,2-propanediol/0.075% of ethylene glycoldiglycidyl ether and 0.075% of aluminum sulfate (each based on polymer)and heat treated at 140° C. in a through air drying cabinet for 2 hours.The performance data are shown in Table 1.

Example 5

A superabsorbent characterized by a pH of 5-5.5, prepared similarly toExample 7 of EP 0 316 792 B1, was admixed on a 20 g scale in a Waringblender (modified attachment for kitchen blender) with a surfacepostcrosslinking solution (sprayed from a 2 ml hypodermic) consisting of2.3% of water/1.2% of 1,2-propanediol/0.2% of ethylene glycol diglycidylether (each based on polymer) and heat treated at 140° C. in a throughair drying cabinet for 1 hour. The performance data are shown in Table1.

Example 6

369.97 of acrylic acid (AA) is admixed with 2.94 g of ETMPTA(ethoxylated trimethylolpropane triacrlyate) (corresponds to 0.06 mol %based on AA). This solution is added with stirring to 184.83 g of a 50%by weight aqueous solution of NaOH (45 mol % based on AA) in 868.17 g ofdeionized water with cooling. The temperature is measured. At the start,AA is rapidly admixed with ETMPTA until the temperature has reachedabout 50° C. Thereafter AA has ETMPTA (up to about 75%) added to itdropwise so that the temperature remains at about 50° C.+/−5° C. Theremaining AA can be rapidly admixed with ETMPA. The monomer solution iscooled to 10° C. in an ice bath and admixed with 0.09 g of Darocur 1173:Irgacure 651 (2:1 ratio) photoinitiators, which are dissolved withstirring. The monomer solution is cooled again to 10° C. in an ice bathand admixed with 6.832 g of 10% sodium persulfate solution (0.07 mol %based on AA). The monomer solution is added ,in a 1.5 L glass vesselwith temperature control. The vessel is placed under a UV emission lamp(20 mWcm⁻² measured at vessel base). An exothermic polymerization ismeasured for 7.5 minutes, after which a transparent gel can be removedfor further processing.

1 kg of the SAP gel obtained were comminuted for 30 s in a 2 L twinsigma blade mixer to enlarge the surface area and mixed with 100 g offine SAP powder (particle size <106 um) for a further 90 s. A solutionof 26 g of 1% sodium metabisulfite (0.026 mol % based on AA) was addedand mixed in for a further 90 s. This was followed by the addition of 20g of 1% Span 20 solution (sorbitan monolaurate) (0.02% by weight basedon gel) and mixed in for 15 s.

The comminuted gel is extruded in an extruder having a 4 mm die plate toenlarge the surface area and subsequently dried on a metal plate having3 mm holes for air circulation at a gel thickness of 2-4 mm at 160° C.for 45 min.

The dried material is comminuted in a hammer mill and the particle sizeis adjusted by sieving to 180-710 μm.

The subsequent surface crosslinking is carried out with 4% by weight ofa solution containing 1.5% of ethylene glycol diglycidyl ether(EGDGE—Nagase Chemicals Japan as Denacol Ex-810), 36.94% of propyleneglycol and 61.56% of water by drying and crosslinking at 145° C. for 60min.

Example 7

Similar to Example 6 except that 205.37 g of 50% NaOH solution in 847.63g of water are introduced as initial charge.

The performance data are shown in Table 1.

Example 8a

A Werner & Pfleiderer laboratory kneader having a working capacity of 2l is evacuated to 50 mbar by means of vacuum and a previously separatelyprepared monomer solution which has been cooled to about 25° C. andinertized by passing nitrogen into it is sucked into the kneader. Themonomer solution has the following composition: 3095.7 g of completelyion-free water, 1821.68 g of acrylic acid, 1012-05 g of NaOH 50%, 8.20 gof polyethylene glycol 400 diacrylate (Sartomer® 344 from CRAY VALLEY)and also 1.46 g of sorbitan monococoate. To improve the inertization,the kneader is evacuated and subsequently refilled with nitrogen. Thisoperation is repeated 3 times. Then 35.22 g of an aqueous 15% sodiumpersulfate solution, 0.18 g of a 1% aqueous hydrogen peroxide solutionand, 30 seconds later, 25.50 g of a 0.5% aqueous ascorbic acid solutionis sucked in. After a nitrogen purge a preheated jacket heating circuiton bypass at 75° C. is switched over to the kneader jacket and thestirrer speed raised to 96 rpm. Following the onset of polymerizationand the attainment of T_(max), the jacket heating circuit is switchedback to bypass, and the batch is supplementarily polymerized for 15minutes without heating/cooling, subsequently cooled and discharged. Theresultant gel particles are dried at 160° C. on wire mesh bottomed traysin a through air drying cabinet and then ground and sieved.

Example 8b

1200 g of a thus obtained product of particle size distribution 105-850μm were sprayed with a homogeneous solution consisting of 39.5 g ofwater, 19.7 g of 1,2-propanediol and 0.72 g of ethylene glycoldiglycidyl ether in a powder mixing assembly (Loedige mixer) andtransferred into a second, preheated Loedige mixer. The heat treatmentwas carried out under constant conditions at a jacket temperature of150° C. and a speed of 60 rpm for a period of 120 minutes. The mixer wasemptied, and the product was cooled down to room temperature and sievedoff at 105-850 μm to remove any agglomerates or fines which may haveformed. The performance data are shown in Table 1.

Example 9

Superabsorbents as described in Example 1 of WO 00/22018 page 14.

TABLE 1 CRC AUL 0.7 psi Example pH SFC × 10⁻⁷ cm³s/g g/g g/g pH_(AI) 1a4.44 1 23.2 5.8 74.2 1b 4.45 14.5 21.4 14.5 91.5 1c 4.47 13.8 20.7 18.198.2 1d 4.45 12 21.9 14.7 93.3 2 4.39 1 27.6 5.0 85.1 2a 4.39 16 25.113.0 99.4 2b 4.39 24 23.3 15.8 102 2c 4.39 33 22.4 18.1 105.7 3 4.68 132.4 4.9 86.5 3a 4.69 9 29.0 13.9 99 4 4.37 2 30.0 4.8 91.5 4a 4.39 3122.4 15.7 99.4 Example 7 of 5.4 ≦1 42.0 6.0 76.8 EP 316 792 notinventive 5 5.4 17 32.9 23.0 89.4 6 5.1 18 29.7 26.4 106.6 7 5.3 16 30.726.8 97.8 8 5.2 28.2 26.1 97.7

Table 2 summarizes the experimental results of the ammonia nitrogendetermination of the products having different pH values from theexamples. Table 2 demonstrates the odor binding effect of acidicsuperabsorbent products.

TABLE 2 N₂ from NH₃ (Nessler) Nessler Example pH mg/l value  1c 4.47 1.820% 6 5.1 2.1 23% 7 5.3 3.9 43% 5 5.4 5.6 62% 9 6.1 9.0 100%

1. A hydrogel-forming polymer capable of absorbing aqueous fluids andhaving a pH absorbency index pH_(AI) of at least 90 and a pH of not morethan 5.7, wherein pH_(AI) is calculated as follows: pH_(AI)=ΔpH(AUL_(0.7 psi)+CRC), wherein ΔpH=7-pH of product, AUL _(0.7 psi) is anabsorbency under pressure at 0.7 psi, and CRC is a centrifuge retentioncapacity.
 2. The polymer of claim 1 having a pH absorbency index pH_(AI)of at least
 100. 3. The polymer of claim 1 having a pH of from 4 to 5.5.4. The polymer of claim 1 having a pH of from 4.4 to 4.6 or from 5.1 to5.3.
 5. The polymer of claim 1 having a Nessler value of less than 60%.6. The polymer of claim 1 having a Nessler value of less than 40%. 7.The polymer of claim 1 having a Nessler value of less than 30%.
 8. Thepolymer of claim 1 having a CRC greater than 15 g/g.
 9. The polymer ofclaim 1 having a CRC greater than 26 g/g.
 10. The polymer of claim 1having an AUL_(0.7 psi) greater than 13 g/g.
 11. The polymer of claim 1having an AUL_(0.7 psi) greater than 15 g/g.
 12. The polymer of claim 1obtainable from a base polymer having a pH of not more than 5.7.
 13. Thepolymer of claim 1 being an acrylic acid (co)polymer partiallyneutralized with an alkali metal, an alkaline earth metal, ammonium, oran amine salt.
 14. The polymer of claim 1 wherein the polymer is surfacepostcrosslinked.
 15. The polymer of claim 1 having antimicrobialproperties.
 16. The polymer of claim 1 prepared by polymerizating apartially neutralized acrylic acid with a crosslinking agent to form agel, wherein (i) partial neutralization is effected using 50% NaOH or anequivalent neutralizing agent: 20-30 mol% based on acrylic acid, andcrosslinking is effected using polyethylene glycol 400 diacrylate:0.005-1.0%, by weight, based on acrylic acid, or a different crosslinkerthat produces the same degree of crosslinking, or (ii) partialneutralization is effected using NaHCO₃ or an equivalent neutralizingagent: 23-28%, by weight, based on acrylic acid, and crosslinking iseffected using allyl methacrylate: 0.005-1.0%, by weight, based onacrylic acid, or a different crosslinker that produces the same degreeof crosslinking, or (iii) partial neutralization is effected using LiOH×H₂O or an equivalent neutralizing agent: 15-30%, by weight, based onacrylic acid, and crosslinking is effected using allyl methacrylate:0.005-1.0%, by weight, based on acrylic acid, or a different crosslinkerthat produces the same degree of crosslinking, or (iv) partialneutralization is effected using NaHCO₃ or an equivalent neutralizingagent: 35-55%, by weight, based on acrylic acid, when2-acrylamido-2-methylpropanesulfonic acid is present at 30-55%, byweight, based on acrylic acid, and crosslinking is effected using allylmethacrylate: 0.005-1.0%, by weight, based on acrylic acid, or adifferent crosslinker that produces the same degree of crosslinking. 17.A base polymer obtained by drying a gel prepared in claim
 16. 18. Apolymer obtained by surface postcrosslinking the dried base polymer ofclaim
 17. 19. A hygiene article comprising (A) a liquid pervioustopsheet, (B) a liquid impervious backsheet, (C) a core positionedbetween (A) and (B), said core comprising 10% to 100%, by weight, of thehydrogel-forming polymer of claims 1 and 0% to 90%, by weight, ofhydrophilic fiber material, (D) optionally a tissue layer positioneddirectly above and below said core (C), and (E) optionally anacquisition layer positioned between (A) and (C).